The success of bypass grafting for popliteal aneurysms may be compromised by the natural history of these lesions. During a 9-year period 35 patients had repair of 48 popliteal aneurysms. Elective repair was performed in 26 limbs and the remaining 22 limbs required urgent or emergent treatment after aneurysm thrombosis (16) or embolic digital ischemia (six). Retrospective review of preoperative and intraoperative arteriograms and CT or ultrasound scans was done to define the relationship of distal runoff anatomy to initial symptoms and long-term grafting success during a mean follow-up of 48 months. Only five of 48 limbs (10%) had three continuous tibial vessels and a patent pedal arch. Twenty-six limbs (55%) had one or no named calf vessels and 11 limbs had incomplete pedal arches. When thrombosis or symptomatic embolism occurred, the anatomy was significantly worse; 15 of 22 such limbs (68%) had one or no patent tibial vessels and seven had incomplete pedal arches. Nineteen of 28 aneurysms examined by CT or ultrasound scanning showed intraluminal thrombus. Eight of 13 patent aneurysms with intraluminal thrombus had one or no patent tibial vessels. The 5-year graft patency rate was 74% for the entire study group. Bypass for repair of asymptomatic aneurysms had a patency rate of 91% at 5 years vs 54% for symptomatic patients (p less than 0.05). Patients with popliteal aneurysms often have advanced tibial disease that appears worse in those with symptoms. The occurrence of a patent popliteal aneurysm with intraluminal thrombus and advanced runoff disease suggests that chronic microembolism may be an etiologic factor in the tibial disease observed.(ABSTRACT TRUNCATED AT 250 WORDS)