Forty-five cases in 33 patients of congenital muscular abnormalities causing vascular compression in the popliteal fossa are reported. Three main types of abnormality were distinguished. In the first, one muscle, either the medial head of gastrocnemius or the plantaris, is abnormal. In the second, both these muscles contribute to the problem, whereas in the third type, the semi-membranosus also is abnormal. In some cases an abnormal course of the popliteal vessel(s) is also found. With only a few exceptions, cases reported in the literature fit into this classification. Twelve patients in this series were affected bilaterally but only 5 had the same anatomy in both legs. In only 14 instances, either with thrombosis or with characteristic shifting of the artery, was routine arteriography sufficient for diagnosis; loss of distal pulses during active plantar flexion was necessary in the majority to demonstrate the abnormality. It is suggested that the condition is more common than has been thought.