One hundred five unselected and consecutive patients were prospectively studies after acute transmural myocardial infarction to assess the incidence of mural thrombus formation and to relate the presence of thrombus to patient outcome in terms of systemic embolic events, functional class and survival. In 87 patients, optimal quality two-dimensional echocardiographic studies were obtained and were repeated at daily intervals to detect mural thrombus formation. The site of infarction was anterior in 53 patients and inferior in 34. On admission, all patients received subcutaneous heparin and antiplatelet agents (aspirin, dipyridamole); none received full anticoagulant therapy. Left ventricular mural thrombus was visualized between 2 and 11 days (median 6) after the clinical onset of infarction in 21 (40%) of the 53 patients with anterior infarction. No patients with inferior infarction had echocardiographic evidence of thrombus formation. During follow-up of 22 to 51 months (mean 39), none of the 21 patients with mural thrombus had clinical evidence of systemic embolism. One patient with inferior and one with anterior infarction had a cerebral embolus 7 days and 9 months, respectively, after the acute event, but neither of these patients had echocardiographic evidence of left ventricular thrombus at any stage. Echocardiography performed at 1 and 2 years of follow-up showed persistent evidence of thrombus in only 8 (31%) and 5 (24%) of the 21 patients, respectively. On admission, the functional class of patients with anterior myocardial infarction and thrombus was similar to that of patients without ventricular thrombus.(ABSTRACT TRUNCATED AT 250 WORDS)