Adventitial cystic disease of the popliteal artery has been treated surgically in two patients. Both patients were young men, otherwise healthy, in whom disabling calf claudication developed. Arteriography demonstrated a crescentic defect in one patient and near-total midpopliteal occlusion in the other. Resection and a bypass graft resulted in complete relief of symptoms. This disease should be considered when examining a young person with claudication. The differential diagnosis of claudication in a young adult is primarily between this entity and popliteal artery entrapment syndrome.