diff --git a/moto/swf/models/__init__.py b/moto/swf/models/__init__.py
index c8cb7916..19ec6eb4 100644
--- a/moto/swf/models/__init__.py
+++ b/moto/swf/models/__init__.py
@@ -287,6 +287,17 @@ class SWFBackend(BaseBackend):
         else:
             return None
 
+    def count_pending_activity_tasks(self, domain_name, task_list):
+        self._check_string(domain_name)
+        self._check_string(task_list)
+        domain = self._get_domain(domain_name)
+        count = 0
+        for _task_list, tasks in domain.activity_task_lists.iteritems():
+            if _task_list == task_list:
+                pending = [t for t in tasks if t.state in ["SCHEDULED", "STARTED"]]
+                count += len(pending)
+        return count
+
 
 swf_backends = {}
 for region in boto.swf.regions():
diff --git a/moto/swf/responses.py b/moto/swf/responses.py
index 66493aef..890b2357 100644
--- a/moto/swf/responses.py
+++ b/moto/swf/responses.py
@@ -242,7 +242,6 @@ class SWFResponse(BaseResponse):
         count = self.swf_backend.count_pending_decision_tasks(domain_name, task_list)
         return json.dumps({"count": count, "truncated": False})
 
-
     def respond_decision_task_completed(self):
         task_token = self._params["taskToken"]
         execution_context = self._params.get("executionContext")
@@ -265,3 +264,9 @@ class SWFResponse(BaseResponse):
             )
         else:
             return json.dumps({"startedEventId": 0})
+
+    def count_pending_activity_tasks(self):
+        domain_name = self._params["domain"]
+        task_list = self._params["taskList"]["name"]
+        count = self.swf_backend.count_pending_activity_tasks(domain_name, task_list)
+        return json.dumps({"count": count, "truncated": False})
diff --git a/tests/test_swf/responses/test_activity_tasks.py b/tests/test_swf/responses/test_activity_tasks.py
index ea95c556..a4fb8684 100644
--- a/tests/test_swf/responses/test_activity_tasks.py
+++ b/tests/test_swf/responses/test_activity_tasks.py
@@ -44,3 +44,28 @@ def test_poll_for_activity_task_on_non_existent_queue():
     resp = conn.poll_for_activity_task("test-domain", "non-existent-queue")
     resp.should.equal({"startedEventId": 0})
 
+
+# CountPendingActivityTasks endpoint
+@mock_swf
+def test_count_pending_activity_tasks():
+    conn = setup_workflow()
+    decision_token = conn.poll_for_decision_task("test-domain", "queue")["taskToken"]
+    conn.respond_decision_task_completed(decision_token, decisions=[
+        {
+            "decisionType": "ScheduleActivityTask",
+            "scheduleActivityTaskDecisionAttributes": {
+                "activityId": "my-activity-001",
+                "activityType": { "name": "test-activity", "version": "v1.1" },
+                "taskList": { "name": "activity-task-list" },
+            }
+        }
+    ])
+
+    resp = conn.count_pending_activity_tasks("test-domain", "activity-task-list")
+    resp.should.equal({"count": 1, "truncated": False})
+
+@mock_swf
+def test_count_pending_decision_tasks_on_non_existent_task_list():
+    conn = setup_workflow()
+    resp = conn.count_pending_activity_tasks("test-domain", "non-existent")
+    resp.should.equal({"count": 0, "truncated": False})