Adventitial cystic disease of the popliteal artery is an important cause of peripheral vascular insufficiency in the young and middle-aged man. The pathologic feature is a mucinous cyst located within the adventitia of the artery that expands and secondarily compromises the vessel lumen. Although physiologically quite different, this process is easily mistaken for arteriosclerosis. The clinical history of sudden claudication in a young nonsmoking man, combined with characteristic angiographic features, are important clues to the correct underlying pathology. Treatment generally consists of cyst evacuation or local bypass. This article was prompted by the failure of percutaneous transluminal angioplasty to achieve durable success in controlling this unique type of arterial disease. Subsequent surgical intervention proved satisfactory, lending support to this modality as the treatment of choice.