diff --git a/src/core.ts b/src/core.ts
index 3591728..c837dc6 100644
--- a/src/core.ts
+++ b/src/core.ts
@@ -349,7 +349,11 @@ export abstract class APIClient {
       delete reqHeaders['content-type'];
     }
 
-    reqHeaders['x-stainless-retry-count'] = String(retryCount);
+    // Don't set the retry count header if it was already set or removed by the caller. We check `headers`,
+    // which can contain nulls, instead of `reqHeaders` to account for the removal case.
+    if (getHeader(headers, 'x-stainless-retry-count') === undefined) {
+      reqHeaders['x-stainless-retry-count'] = String(retryCount);
+    }
 
     this.validateHeaders(reqHeaders, headers);
 
@@ -1128,7 +1132,15 @@ export const isHeadersProtocol = (headers: any): headers is HeadersProtocol => {
   return typeof headers?.get === 'function';
 };
 
-export const getRequiredHeader = (headers: HeadersLike, header: string): string => {
+export const getRequiredHeader = (headers: HeadersLike | Headers, header: string): string => {
+  const foundHeader = getHeader(headers, header);
+  if (foundHeader === undefined) {
+    throw new Error(`Could not find ${header} header`);
+  }
+  return foundHeader;
+};
+
+export const getHeader = (headers: HeadersLike | Headers, header: string): string | undefined => {
   const lowerCasedHeader = header.toLowerCase();
   if (isHeadersProtocol(headers)) {
     // to deal with the case where the header looks like Stainless-Event-Id
@@ -1154,7 +1166,7 @@ export const getRequiredHeader = (headers: HeadersLike, header: string): string
     }
   }
 
-  throw new Error(`Could not find ${header} header`);
+  return undefined;
 };
 
 /**
diff --git a/tests/index.test.ts b/tests/index.test.ts
index 28469d4..fb1e7c9 100644
--- a/tests/index.test.ts
+++ b/tests/index.test.ts
@@ -266,6 +266,64 @@ describe('retries', () => {
     expect(count).toEqual(3);
   });
 
+  test('omit retry count header', async () => {
+    let count = 0;
+    let capturedRequest: RequestInit | undefined;
+    const testFetch = async (url: RequestInfo, init: RequestInit = {}): Promise<Response> => {
+      count++;
+      if (count <= 2) {
+        return new Response(undefined, {
+          status: 429,
+          headers: {
+            'Retry-After': '0.1',
+          },
+        });
+      }
+      capturedRequest = init;
+      return new Response(JSON.stringify({ a: 1 }), { headers: { 'Content-Type': 'application/json' } });
+    };
+    const client = new RunwayML({ apiKey: 'My API Key', fetch: testFetch, maxRetries: 4 });
+
+    expect(
+      await client.request({
+        path: '/foo',
+        method: 'get',
+        headers: { 'X-Stainless-Retry-Count': null },
+      }),
+    ).toEqual({ a: 1 });
+
+    expect(capturedRequest!.headers as Headers).not.toHaveProperty('x-stainless-retry-count');
+  });
+
+  test('overwrite retry count header', async () => {
+    let count = 0;
+    let capturedRequest: RequestInit | undefined;
+    const testFetch = async (url: RequestInfo, init: RequestInit = {}): Promise<Response> => {
+      count++;
+      if (count <= 2) {
+        return new Response(undefined, {
+          status: 429,
+          headers: {
+            'Retry-After': '0.1',
+          },
+        });
+      }
+      capturedRequest = init;
+      return new Response(JSON.stringify({ a: 1 }), { headers: { 'Content-Type': 'application/json' } });
+    };
+    const client = new RunwayML({ apiKey: 'My API Key', fetch: testFetch, maxRetries: 4 });
+
+    expect(
+      await client.request({
+        path: '/foo',
+        method: 'get',
+        headers: { 'X-Stainless-Retry-Count': '42' },
+      }),
+    ).toEqual({ a: 1 });
+
+    expect((capturedRequest!.headers as Headers)['x-stainless-retry-count']).toBe('42');
+  });
+
   test('retry on 429 with retry-after', async () => {
     let count = 0;
     const testFetch = async (url: RequestInfo, { signal }: RequestInit = {}): Promise<Response> => {