diff --git a/requestwrite_test.go b/requestwrite_test.go
index 53dee7bd..96d6bcdc 100644
--- a/requestwrite_test.go
+++ b/requestwrite_test.go
@@ -12,6 +12,7 @@ import (
 	"io"
 	"net"
 	"net/url"
+	"slices"
 	"strings"
 	"testing"
 	"testing/iotest"
@@ -978,3 +979,49 @@ func (c *dumpConn) RemoteAddr() net.Addr               { return nil }
 func (c *dumpConn) SetDeadline(t time.Time) error      { return nil }
 func (c *dumpConn) SetReadDeadline(t time.Time) error  { return nil }
 func (c *dumpConn) SetWriteDeadline(t time.Time) error { return nil }
+
+// The transfer headers take their values from the request but keep the name
+// the caller wrote.
+func TestRequestWriteKeepsTransferHeaderNames(t *testing.T) {
+	tests := []struct {
+		name    string
+		method  string
+		body    io.Reader
+		length  int64
+		header  Header
+		trailer Header
+		want    []string // transfer header lines, in order
+	}{
+		{"lowercase", "POST", strings.NewReader("abc"), 3, Header{"content-length": {"3"}}, nil, []string{"content-length: 3"}},
+		{"uppercase with a wrong value", "POST", strings.NewReader("abc"), 3, Header{"CONTENT-LENGTH": {"99"}}, nil, []string{"CONTENT-LENGTH: 3"}},
+		{"two spellings", "POST", strings.NewReader("abc"), 3, Header{"content-length": {"3"}, "Content-length": {"3"}}, nil, []string{"Content-Length: 3"}},
+		{"GET drops it", "GET", nil, 0, Header{"content-length": {"3"}}, nil, nil},
+		{"lowercase transfer-encoding", "POST", io.MultiReader(strings.NewReader("abc")), -1, Header{"transfer-encoding": {"chunked"}}, nil, []string{"transfer-encoding: chunked"}},
+		{"lowercase trailer", "POST", io.MultiReader(strings.NewReader("abc")), -1, Header{"trailer": {"x"}}, Header{"X-Foo": nil}, []string{"Transfer-Encoding: chunked", "trailer: X-Foo"}},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			req := dummyRequest(tt.method)
+			req.Header, req.ContentLength, req.Trailer = tt.header, tt.length, tt.trailer
+			if tt.body != nil {
+				req.Body = io.NopCloser(tt.body)
+			}
+			var buf bytes.Buffer
+			if err := req.Write(&buf); err != nil {
+				t.Fatal(err)
+			}
+			head, _, _ := strings.Cut(buf.String(), "\r\n\r\n")
+			var got []string
+			for _, line := range strings.Split(head, "\r\n")[1:] {
+				name, _, _ := strings.Cut(line, ":")
+				switch strings.ToLower(name) {
+				case "content-length", "transfer-encoding", "trailer":
+					got = append(got, line)
+				}
+			}
+			if !slices.Equal(got, tt.want) {
+				t.Errorf("transfer headers %q, want %q\n%s", got, tt.want, head)
+			}
+		})
+	}
+}
diff --git a/transfer.go b/transfer.go
index bdb30abe..9e24cdcb 100644
--- a/transfer.go
+++ b/transfer.go
@@ -275,6 +275,8 @@ func (t *transferWriter) shouldSendContentLength() bool {
 // addHeaders adds transfer headers to an existing header object, replacing any the caller set with the same name.
 // Content-Length always comes from the request, as in net/http.
 func (t *transferWriter) addHeaders(hdrs *Header) error {
+	// Not setTransferHeader: the caller's Content-Length goes even when none is sent.
+	contentLength := headerNameFold(*hdrs, "Content-Length")
 	deleteHeaderFold(*hdrs, "Content-Length")
 	if t.Close && !hasToken(t.Header.get("Connection"), "close") {
 		hdrs.Add("Connection", "close")
@@ -284,7 +286,7 @@ func (t *transferWriter) addHeaders(hdrs *Header) error {
 	// function of the sanitized field triple (Body, ContentLength,
 	// TransferEncoding)
 	if t.shouldSendContentLength() {
-		setTransferHeader(*hdrs, "Content-Length", strconv.FormatInt(t.ContentLength, 10))
+		(*hdrs)[contentLength] = []string{strconv.FormatInt(t.ContentLength, 10)}
 	} else if chunked(t.TransferEncoding) {
 		setTransferHeader(*hdrs, "Transfer-Encoding", "chunked")
 	}
@@ -311,10 +313,30 @@ func (t *transferWriter) addHeaders(hdrs *Header) error {
 	return nil
 }
 
-// setTransferHeader sets key to value in h, removing the caller's value under any case of key.
+// setTransferHeader sets key to value in h, replacing the caller's value under
+// any case of key but keeping the name the caller wrote.
 func setTransferHeader(h Header, key, value string) {
+	name := headerNameFold(h, key)
 	deleteHeaderFold(h, key)
-	h.Set(key, value)
+	h[name] = []string{value}
+}
+
+// headerNameFold returns the name under which h has key, if h has it under
+// exactly one case, and otherwise the canonical form of key.
+func headerNameFold(h Header, key string) string {
+	name := ""
+	for k := range h {
+		if strings.EqualFold(k, key) {
+			if name != "" {
+				return CanonicalHeaderKey(key)
+			}
+			name = k
+		}
+	}
+	if name == "" {
+		return CanonicalHeaderKey(key)
+	}
+	return name
 }
 
 // deleteHeaderFold deletes key from h under any case.