1. 1. A case of occlusion of the popliteal artery with thrombosis and aneurysmal formation distal to entrapment by an abnormal laterally attached medial head of the gastrocnemius muscle is reported. 2. 2. Compression of the popliteal vein also by the abnormally situated medial head of the gastrocnemius muscle has not been reported in other cases reviewed. 3. 3. Entrapment of the popliteal artery is probably more common than the sparse reports in the literature would indicate, but to diagnose the syndrome one must be aware of its existence. 4. 4. Medial deviation of the popliteal artery seen on the femoral arteriogram should alert one to the possible presence of this anomaly. 5. 5. Several authors have stated that this diagnosis should be suspected in young male patients with unilateral calf claudication; however, in some instances this syndrome may not be evidenced until later years.