This study evaluated the influence of thrombolysis followed by full anticoagulation on the frequency of left ventricular (LV) thrombi after acute myocardial infarction (AMI). Nineteen consecutive patients with a first anterior wall AMI who received 1,500,000 IU of streptokinase within 3 hours of symptom onset, followed by full anticoagulation, underwent echocardiographic studies within 24 hours of symptoms, and then on days 2, 3, 5, 7, 12, 30 and 90. Forty-four patients, with comparable clinical features and echocardiographic protocol but without antithrombotic therapy, served as the control group. LV thrombi developed in 4 of 19 (21%) treated patients and in 23 of 44 (52%) control subjects (p = 0.02). LV aneurysm or major wall motion abnormalities were noted in 8 of 19 (42%) treated patients and in 30 of 44 (68%) control subjects (p less than 0.05). No significant difference was found between treated and untreated patients when comparing the incidence of thrombi in the subgroups of patients with aneurysm or major wall motion abnormalities (3 of 8 vs 21 of 30) and in the subgroups with less extensive LV dysfunction. Thrombi disappeared during hospitalization in 3 of 4 treated patients, but in none of the controls. Fewer patients treated with intravenous streptokinase followed by full anticoagulation developed LV thrombi compared to patients treated with conventional therapy. This difference may be related to a reduced occurrence of major LV wall motion abnormalities. Resolution of thrombi frequently occurs in the hospital phase of AMI; therefore, only frequent echocardiographic examinations can assess the true frequency of LV thrombi.