Between 1960--1979, 19 patients with aortofemoral prosthetic bypass for aneurysmal and/or occlusive disease subsequently developed 36 false aneurysms at the suture line, for an incidence of 4.5% (19/426 patients). The two major aneurysm sites were the femoral anastomosis 4.5% (33/727) patients), and the aortic anastomosis 0.7% (3/430 patients). Additionally, one patient with bilateral false aneurysms of the groin subsequently developed an aortoduodenal fistula. The initial operation was for aortoiliac occlusive disease in 14 and for abdominal aortic aneurysms plus severe occlusive disease in five. The false aneurysm(s) appeared three months to 17 years after the aortofemoral procedure. Eleven of 19 patients (57.9%) had multiple aneurysms (two to five) and developed both right and left groin aneurysms concomitantly or at different times. Suture failure was the major finding at operation. However, a common alternative finding was partial or near complete separation of the prosthesis from the host vessel and an intact suture line, thus indicating a structural weakness in the host vessel and/or severe mechanical stress, as the causative factor. One experience leads us to favor an aggressive surgical approach to these lesions. All false aneurysms were corrected as they were detected with good surgical results.