In a consecutive series of 275 surgically treated abdominal aortic aneurysms, seven patients with acute thrombotic occlusion of an abdominal aortic or iliac aneurysm were encountered. Ten additional cases have been reported in the medical literature. The majority of the aneurysms were 7 cm in diameter or smaller. Some patients had complained of claudication before the acute event. An acute low-flow state superimposed on a stenotic atherosclerotic distal vascular bed is probably the main factor responsible for acute thrombosis within an abdominal aneurysm. The high mortality of 53% is similar to that of ruptured abdominal aortic aneurysms. The recommendation is made that all small abdominal aortic aneurysms be resected if there is substantial associated distal occlusive disease.