Popliteal artery aneurysms. Long-term follow-up of aneurysmal disease and results of surgical treatment. The natural history of aneurysmal disease was analyzed in 50 patients who were treated for 71 popliteal aneurysms. No patients were lost to follow-up (mean, 5.0 years). Initially, 25 popliteal aneurysms (25/71; 35%) were treated nonsurgically, and 46 (46/71; 65%) were treated surgically. Complications developed in 12 of the 21 asymptomatic popliteal aneurysms (57%) and in 2 of the 4 symptomatic popliteal aneurysms (50%), which were treated nonsurgically. The probability of developing complications increased with time to 74% within 5 years. When reconstruction of a popliteal aneurysm was performed, graft patency and foot salvage were 64% and 95% at 10 years, respectively. Particularly acute arterial thromboembolism was a severe presenting complication. Another important finding was the development of 23 arteriosclerotic aneurysms at other locations during follow-up in 16 patients (32%). The probability of developing these new aneurysms increased to 49% 10 years after repair of the initial popliteal aneurysm. The presence of multiple isolated aneurysms at the initial examination was the most significant risk factor limiting the survival of these patients. Consequently patients at risk could be identified early. This study confirms the limb-threatening potential of popliteal aneurysms when left untreated. Therefore prophylactic reconstructive surgery should be undertaken. Moreover, this study demonstrates that patients with a popliteal artery aneurysm have an increased risk of new aneurysm formation, both in the popliteal artery and at other locations. Therefore these patients should be followed and, in the event that new aneurysms develop, should be considered for elective reconstructive surgery to prevent limb-threatening or life-endangering complications. 