BACKGROUND Limited data exist concerning left ventricular thrombi during and after hospitalization in patients treated according to modern principles. The purpose of the present study was to examine the formation and resolution of left ventricular thrombi during the first month in patients with acute anterior myocardial infarction treated with streptokinase and aspirin. METHODS Seventy-seven consecutive patients were studied prospectively during the hospital stay and 1-month follow-up study. Aspirin was used routinely, whereas anticoagulants were only used after a decision by the attending physician. Echocardiography was performed within 3 days of admission, before hospital discharge and after 1 month of follow-up. RESULTS At the first examination, 17 of 77 patients (22%) had a thrombus. At discharge, 73 patients remained in the study. In five (31%) of the 16 patients with early thrombus, the thrombus persisted; in 18 (32%) of the 57 patients without early thrombus, a new thrombus was diagnosed. One month later, 65 patients remained eligible for follow-up study. In three of 20 patients (15%) the thrombus from the second examination persisted and in four of 45 patients (9%) a new thrombus was diagnosed. The disappearance rate between the second and third examination was high irrespective of whether patients were treated with anticoagulants (eight of nine, 89%) or not (nine of 11, 82%). Extensive left ventricular segmental dysfunction and signs of congestive heart failure were associated with the appearance of a left ventricular thrombus. No embolic events were recorded. CONCLUSIONS In patients with anterior myocardial infarction treated with streptokinase and aspirin the development and disappearance of left ventricular thrombi is a highly dynamic process. A large proportion of thrombi resolve without additional anticoagulant therapy.