A 62-year-old man with intermittent claudication but normal foot pulses at rest had no atheroma shown on femoral arteriography. There was evidence of external compression of the artery at the level of a popliteal cyst which was demonstrated by arthrography on films taken after sufficient exercise to be in communication with the knee joint. At operation the cyst was continuous with changes of cystic adventitial disease of the popliteal artery. Both the main cyst and th cystic changes in the artery were removed. Postoperatively the patient's claudication disappeared. Evidence is presented that synovial cysts play a part in the production of arterial cystic adventitial disease.