To determine the prevalence and natural history of left ventricular thrombus in dilated cardiomyopathy, we prospectively performed two-dimensional echocardiograms in 25 patients with nonischemic dilated cardiomyopathy who were not receiving anticoagulation. Eighty-five echocardiograms were performed serially over a 9- to 30-month period (mean follow-up 21.5 months). A left ventricular thrombus was present on initial echocardiogram in 11 (44%) patients, became present during follow-up in an additional four, and disappeared in two. Thrombus was significantly more common in patients with fractional shortening of less than or equal to 10% (12 of 15) than in those with a fractional shortening 11% to 25% (3 of 10) (p less than 0.02). Five embolic events (four cerebral) occurred over the follow-up period, four of which were associated with a previously visualized left ventricular thrombus. Three of five thrombi that protruded into the left ventricular cavity subsequently embolized. We conclude that in nonanticoagulated patients with dilated cardiomyopathy left ventricular thrombus and thromboembolism are common. Echocardiography may be helpful in predicting which patients are at risk of thromboembolism.