Transesophageal echocardiography was used to assess cardiac abnormalities associated with embolization in patients who had completed the Department of Veterans Affairs Cooperative Study of Stroke Prevention in Nonrheumatic Atrial Fibrillation at the Minneapolis and West Haven Department of Veterans Affairs Medical Centers without an embolic event. Patients were men, 71 +/- 7 years old, with atrial fibrillation of 6.2 +/- 4.3 years' duration who had received warfarin (n = 32) or placebo (n = 23) for 2 years. Thrombi were found in 5 of 55 patients (warfarin 4 and placebo 1; p = 0.39); spontaneous echo contrast was seen in 4 of 5 patients. Other abnormalities identified included spontaneous echo contrast (47%), patent foramen ovale (54%), atrial septal aneurysm (7.3%), and left ventricular thrombus (3.6%). During 34 months of posttreatment follow-up, 5 patients had a stroke (1 fatal), and 10 died. Potential sources of emboli did not predict subsequent outcome. Thus warfarin therapy did not preclude the presence of thrombi. Stroke reduction likely involves the prevention of emboli from sources in addition to the atrial appendage.