diff --git a/python_multipart/multipart.py b/python_multipart/multipart.py
index ace4a8f..8401f67 100644
--- a/python_multipart/multipart.py
+++ b/python_multipart/multipart.py
@@ -15,7 +15,7 @@
 from .exceptions import FileError, FormParserError, MultipartParseError, QuerystringParseError
 
 if TYPE_CHECKING:  # pragma: no cover
-    from typing import Any, Callable, Literal, Protocol, TypedDict
+    from typing import Any, Callable, Literal, Mapping, Protocol, TypedDict
 
     from typing_extensions import TypeAlias
 
@@ -1617,7 +1617,8 @@ def _on_end() -> None:
 
             header_name: list[bytes] = []
             header_value: list[bytes] = []
-            headers: dict[bytes, bytes] = {}
+            # Header keys are always inserted in Title-Case
+            headers: dict[str, bytes] = {}
 
             f_multi: FileProtocol | FieldProtocol | None = None
             writer = None
@@ -1652,7 +1653,9 @@ def on_header_value(data: bytes, start: int, end: int) -> None:
                 header_value.append(data[start:end])
 
             def on_header_end() -> None:
-                headers[b"".join(header_name)] = b"".join(header_value)
+                # Convert header name to title case.
+                header_name_tc = b"".join(header_name).decode().title()
+                headers[header_name_tc] = b"".join(header_value)
                 del header_name[:]
                 del header_value[:]
 
@@ -1662,8 +1665,7 @@ def on_headers_finished() -> None:
                 is_file = False
 
                 # Parse the content-disposition header.
-                # TODO: handle mixed case
-                content_disp = headers.get(b"Content-Disposition")
+                content_disp = headers.get("Content-Disposition")
                 disp, options = parse_options_header(content_disp)
 
                 # Get the field and filename.
@@ -1681,7 +1683,7 @@ def on_headers_finished() -> None:
                 # Parse the given Content-Transfer-Encoding to determine what
                 # we need to do with the incoming data.
                 # TODO: check that we properly handle 8bit / 7bit encoding.
-                transfer_encoding = headers.get(b"Content-Transfer-Encoding", b"7bit")
+                transfer_encoding = headers.get("Content-Transfer-Encoding", b"7bit")
 
                 if transfer_encoding in (b"binary", b"8bit", b"7bit"):
                     writer = f_multi
@@ -1760,7 +1762,7 @@ def __repr__(self) -> str:
 
 
 def create_form_parser(
-    headers: dict[str, bytes],
+    headers: Mapping[str, bytes],
     on_field: OnFieldCallback | None,
     on_file: OnFileCallback | None,
     trust_x_headers: bool = False,
@@ -1804,7 +1806,7 @@ def create_form_parser(
 
 
 def parse_form(
-    headers: dict[str, bytes],
+    headers: Mapping[str, bytes],
     input_stream: SupportsRead,
     on_field: OnFieldCallback | None,
     on_file: OnFileCallback | None,
@@ -1816,7 +1818,8 @@ def parse_form(
     callbacks that will get called whenever a field or file is parsed.
 
     Args:
-        headers: A dictionary-like object of HTTP headers.  The only required header is Content-Type.
+        headers: A dictionary-like object of HTTP headers.  The only required header is Content-Type,
+            in exactly this form if the input dict is case sensitive.
         input_stream: A file-like object that represents the request body. The read() method must return bytestrings.
         on_field: Callback to call with each parsed field.
         on_file: Callback to call with each parsed file.
diff --git a/tests/test_data/http/mixed_case_headers.http b/tests/test_data/http/mixed_case_headers.http
new file mode 100644
index 0000000..6a4e247
--- /dev/null
+++ b/tests/test_data/http/mixed_case_headers.http
@@ -0,0 +1,19 @@
+----boundary
+ConTenT-TypE: text/plain; charset="UTF-8"
+ConTenT-DisPoSitioN: form-data; name=field1
+ConTenT-TransfeR-EncoDinG: base64
+
+VGVzdCAxMjM=
+----boundary
+content-type: text/plain; charset="UTF-8"
+content-disposition: form-data; name=field2
+content-transfer-encoding: base64
+
+VGVzdCAxMjM=
+----boundary
+CONTENT-TYPE: text/plain; charset="UTF-8"
+CONTENT-DISPOSITION: form-data; name=Field3
+CONTENT-TRANSFER-ENCODING: base64
+
+VGVzdCAxMjM=
+----boundary--
\ No newline at end of file
diff --git a/tests/test_data/http/mixed_case_headers.yaml b/tests/test_data/http/mixed_case_headers.yaml
new file mode 100644
index 0000000..d333067
--- /dev/null
+++ b/tests/test_data/http/mixed_case_headers.yaml
@@ -0,0 +1,14 @@
+boundary: --boundary
+expected:
+  - name: field1
+    type: field
+    data: !!binary |
+      VGVzdCAxMjM=
+  - name: field2
+    type: field
+    data: !!binary |
+      VGVzdCAxMjM=
+  - name: Field3
+    type: field
+    data: !!binary |
+      VGVzdCAxMjM=