diff --git a/include/internal/csv_reader.cpp b/include/internal/csv_reader.cpp
index 56c61a00..378cd1f0 100644
--- a/include/internal/csv_reader.cpp
+++ b/include/internal/csv_reader.cpp
@@ -71,18 +71,40 @@ namespace csv {
         return new_format;
     }
 
+    CSV_INLINE void CSVReader::initial_read() {
+        // Issue #337: A single read window may end before the header row when
+        // earlier rows are long, so keep reading until the header is consumed.
+        do {
+            const int trimmed_before = this->header_rows_trimmed_;
+            this->read_scheduler_.run([this] { this->read_csv(this->_chunk_size); });
+            this->read_scheduler_.join();
+            this->read_scheduler_.rethrow_exception_if_any();
+
+            // trim_header() drains the queue until the header is consumed, so an
+            // untrimmed header with no progress means the read produced no rows.
+            if (!this->header_trimmed && !this->parser->eof()
+                && this->header_rows_trimmed_ == trimmed_before) {
+                internals::throw_row_too_large_for_chunk(this->_chunk_size);
+            }
+        } while (!this->header_trimmed && !this->parser->eof());
+    }
+
     CSV_INLINE void CSVReader::trim_header() {
         if (!this->header_trimmed) {
-            for (int i = 0; i <= this->_format.header && !this->records->empty(); i++) {
-                if (i == this->_format.header && this->col_names->empty()) {
+            // The rows up to and including the header may arrive over several
+            // reads, so track progress instead of assuming one read holds them all.
+            while (this->header_rows_trimmed_ <= this->_format.header && !this->records->empty()) {
+                if (this->header_rows_trimmed_ == this->_format.header && this->col_names->empty()) {
                     this->set_col_names(this->records->pop_front());
                 }
                 else {
                     this->records->pop_front();
                 }
+
+                this->header_rows_trimmed_++;
             }
 
-            this->header_trimmed = true;
+            this->header_trimmed = this->header_rows_trimmed_ > this->_format.header;
         }
     }
 
diff --git a/include/internal/csv_reader.hpp b/include/internal/csv_reader.hpp
index 699ae076..3d8e5d6c 100644
--- a/include/internal/csv_reader.hpp
+++ b/include/internal/csv_reader.hpp
@@ -369,6 +369,9 @@ namespace csv {
     private:
         /** Whether or not rows before header were trimmed */
         bool header_trimmed = false;
+
+        /** How many rows up to and including the header have been trimmed */
+        int header_rows_trimmed_ = 0;
         
         /** @name Reader Scheduling: Flags and State */
         ///@{
@@ -386,6 +389,7 @@ namespace csv {
             this->n_cols = other.n_cols;
             this->_n_rows = other._n_rows;
             this->header_trimmed = other.header_trimmed;
+            this->header_rows_trimmed_ = other.header_rows_trimmed_;
             this->_chunk_size = other._chunk_size;
             this->_read_requested = other._read_requested;
             this->read_scheduler_.set_threading_enabled(this->_format.is_threading_enabled());
@@ -397,6 +401,7 @@ namespace csv {
             this->n_cols = 0;
             this->_n_rows = 0;
             this->header_trimmed = false;
+            this->header_rows_trimmed_ = 0;
             this->_read_requested = false;
             this->_chunk_size = internals::CSV_CHUNK_SIZE_DEFAULT;
         }
@@ -421,12 +426,8 @@ namespace csv {
             );
         }
 
-        /** Read initial chunk to get metadata */
-        void initial_read() {
-            this->read_scheduler_.run([this] { this->read_csv(this->_chunk_size); });
-            this->read_scheduler_.join();
-            this->read_scheduler_.rethrow_exception_if_any();
-        }
+        /** Read until the header row is consumed (or EOF) to get metadata */
+        void initial_read();
 
         void trim_header();
         ///@}
diff --git a/include/internal/parser/mmap.cpp b/include/internal/parser/mmap.cpp
index 5dd32b00..feb14ce5 100644
--- a/include/internal/parser/mmap.cpp
+++ b/include/internal/parser/mmap.cpp
@@ -72,11 +72,21 @@ namespace csv {
             // for delimiter/header guessing.
             if (!head_.empty()) {
                 auto head_owner = std::make_shared<std::string>(std::move(head_));
+                const size_t head_start = this->mmap_pos;
                 const size_t length = head_owner->size();
                 this->mmap_pos += length;
 
                 this->finalize_loaded_chunk(*head_owner, head_owner, length, bytes);
-                return;
+
+                // Issue #337: The head is capped at 500KB regardless of chunk_size,
+                // so it may end before the first row does. Only in that case fall
+                // through and map a full read window from the rewound position;
+                // otherwise return so construction doesn't parse an extra window
+                // (CSVReader::initial_read() keeps reading if the header row is
+                // still pending).
+                if (this->eof_ || this->mmap_pos != head_start) {
+                    return;
+                }
             }
 
             // Create memory map
diff --git a/tests/test_edge_cases_large_rows.cpp b/tests/test_edge_cases_large_rows.cpp
index 460b414c..e4191d63 100644
--- a/tests/test_edge_cases_large_rows.cpp
+++ b/tests/test_edge_cases_large_rows.cpp
@@ -236,6 +236,189 @@ TEST_CASE("Edge case: CSV rows larger than default chunk size", "[edge_cases_lar
     }
 }
 
+TEST_CASE("Issue #337 - First row longer than the mmap head buffer", "[issue_337]") {
+    // MmapParser parses its pre-read head buffer (capped at 500KB regardless of
+    // chunk_size) as the first window. When that window held no complete row,
+    // trim_header() ran against an empty queue, marked the header as trimmed,
+    // and the reader silently produced zero columns and zero rows. The stream
+    // path was unaffected because its first window is the head plus a full read.
+    const size_t n_rows = 1000;
+    std::string body;
+    for (size_t i = 0; i < n_rows; ++i) {
+        body += std::to_string(i * 3 + 0) + ','
+              + std::to_string(i * 3 + 1) + ','
+              + std::to_string(i * 3 + 2) + '\n';
+    }
+
+    // Compare without letting Catch2 expand the huge column name on failure.
+    auto require_col_names = [](CSVReader& reader, const std::string& first) {
+        const auto names = reader.get_col_names();
+        REQUIRE(names.size() == 3);
+        REQUIRE(names[0].size() == first.size());
+        REQUIRE((names[0] == first));
+        REQUIRE(names[1] == "B");
+        REQUIRE(names[2] == "C");
+    };
+
+    SECTION("Header row fits in chunk_size") {
+        const std::string long_name = "A" + std::string(600 * 1024, 'x');
+        const std::string data = long_name + ",B,C\n" + body;
+
+        CSVFormat format;
+        format.delimiter(',').header_row(0).chunk_size(2 * 1024 * 1024).speculative_parallel_threads(1);
+
+        auto validate_reader = [&](CSVReader& reader) {
+            require_col_names(reader, long_name);
+
+            size_t i = 0;
+            for (auto& row : reader) {
+                REQUIRE(row.size() == 3);
+                for (size_t col = 0; col < 3; ++col) {
+                    REQUIRE(row[col].get<size_t>() == i * 3 + col);
+                }
+                ++i;
+            }
+            REQUIRE(i == n_rows);
+        };
+
+        SECTION("stream path") {
+            std::istringstream in(data);
+            CSVReader reader(in, format);
+            validate_reader(reader);
+        }
+
+        SECTION("mmap path") {
+            FileGuard cleanup("./tests/data/tmp_issue_337_long_header.csv");
+            {
+                std::ofstream out(cleanup.filename, std::ios::binary);
+                out << data;
+            }
+            CSVReader reader(cleanup.filename, format);
+            validate_reader(reader);
+        }
+    }
+
+    SECTION("Header row after a short preamble row") {
+        // header_row(1): the head window holds the complete preamble row but
+        // not the long header row, so header trimming must resume on a later read.
+        const std::string long_name = "A" + std::string(600 * 1024, 'x');
+        const std::string data = "preamble\n" + long_name + ",B,C\n" + body;
+
+        CSVFormat format;
+        format.delimiter(',').header_row(1).chunk_size(2 * 1024 * 1024).speculative_parallel_threads(1);
+
+        auto validate_reader = [&](CSVReader& reader) {
+            require_col_names(reader, long_name);
+
+            size_t i = 0;
+            for (auto& row : reader) {
+                REQUIRE(row.size() == 3);
+                REQUIRE(row[0].get<size_t>() == i * 3);
+                ++i;
+            }
+            REQUIRE(i == n_rows);
+        };
+
+        SECTION("stream path") {
+            std::istringstream in(data);
+            CSVReader reader(in, format);
+            validate_reader(reader);
+        }
+
+        SECTION("mmap path") {
+            FileGuard cleanup("./tests/data/tmp_issue_337_preamble.csv");
+            {
+                std::ofstream out(cleanup.filename, std::ios::binary);
+                out << data;
+            }
+            CSVReader reader(cleanup.filename, format);
+            validate_reader(reader);
+        }
+    }
+
+    SECTION("Header row starts after the first read window ends") {
+        // header_row(2) with long preamble rows: the first read window ends
+        // inside the header row on both paths, so trim_header() must not mark
+        // the header as trimmed after popping only the preamble rows.
+        // The 700KB window starting at byte 0 (stream) or after "short\n"
+        // (mmap) ends inside the 150KB header row.
+        const std::string preamble = "preamble," + std::string(600 * 1024, 'p') + '\n';
+        const std::string long_name = "A" + std::string(150 * 1024, 'x');
+        const std::string data = "short\n" + preamble + long_name + ",B,C\n" + body;
+
+        CSVFormat format;
+        format.delimiter(',')
+            .header_row(2)
+            .chunk_size(internals::CSV_CHUNK_SIZE_FLOOR + 200 * 1024)
+            .speculative_parallel_threads(1);  // Keep the stream window at chunk_size
+
+        auto validate_reader = [&](CSVReader& reader) {
+            require_col_names(reader, long_name);
+
+            size_t i = 0;
+            for (auto& row : reader) {
+                REQUIRE(row.size() == 3);
+                REQUIRE(row[2].get<size_t>() == i * 3 + 2);
+                ++i;
+            }
+            REQUIRE(i == n_rows);
+        };
+
+        SECTION("stream path") {
+            std::istringstream in(data);
+            CSVReader reader(in, format);
+            validate_reader(reader);
+        }
+
+        SECTION("mmap path") {
+            FileGuard cleanup("./tests/data/tmp_issue_337_late_header.csv");
+            {
+                std::ofstream out(cleanup.filename, std::ios::binary);
+                out << data;
+            }
+            CSVReader reader(cleanup.filename, format);
+            validate_reader(reader);
+        }
+    }
+
+    SECTION("Header row larger than chunk_size still throws") {
+        const std::string data = "A" + std::string(1200 * 1024, 'x') + ",B,C\n" + body;
+
+        CSVFormat format;
+        format.delimiter(',').header_row(0).chunk_size(internals::CSV_CHUNK_SIZE_FLOOR).speculative_parallel_threads(1);
+
+        auto read_all = [](CSVReader& reader) {
+            for (auto& row : reader) { (void)row; }
+        };
+
+        SECTION("stream path") {
+            REQUIRE_THROWS_WITH(
+                [&]() {
+                    std::istringstream in(data);
+                    CSVReader reader(in, format);
+                    read_all(reader);
+                }(),
+                Catch::Matchers::ContainsSubstring("chunk size")
+            );
+        }
+
+        SECTION("mmap path") {
+            FileGuard cleanup("./tests/data/tmp_issue_337_too_long.csv");
+            {
+                std::ofstream out(cleanup.filename, std::ios::binary);
+                out << data;
+            }
+            REQUIRE_THROWS_WITH(
+                [&]() {
+                    CSVReader reader(cleanup.filename, format);
+                    read_all(reader);
+                }(),
+                Catch::Matchers::ContainsSubstring("chunk size")
+            );
+        }
+    }
+}
+
 TEST_CASE("Issue #218 - Infinite read loop detection", "[issue_218]") {
 
     SECTION("Detects when row exceeds chunk size and file doesn't end") {