Left ventricular thrombus may develop both early and late after acute anterior myocardial infarction. To assess the possible prognostic implication of the time of thrombus appearance, 125 patients (87 males; age ranging from 35 to 92 years, mean: 65 +/- 10 years) consecutively admitted to our coronary care unit within 24 h of a first acute anterior myocardial infarction, untreated with antithrombotic drugs, underwent serial two-dimensional echocardiographic studies during hospitalization, then monthly for a follow-up of 1-48 (mean: 23 +/- 16) months among survivors. Left ventricular thrombi, detected in 71 patients (57%), appeared from 1 to 362 (mean: 13 +/- 44) days after acute infarction. In 40 patients (56%), early thrombus development, within 48 h of symptom onset, was noted. During the study period, 52 patients (42%) died. Global mortality rate was similar in patients with thrombi compared with those without thrombi (32/71: 45%, vs 20/54: 37%; P = ns). However, in-hospital mortality of patients who developed left ventricular thrombi within 48 h (17/40: 42.5%) was significantly higher compared with both patients with later thrombus appearance (4/31: 13%; P less than 0.008) and those without thrombi (10/54: 20%; P less than 0.01). Embolic events were more frequent in patients with thrombi (9/71, 13% vs 1/54, 2%; P less than 0.02), but there was no relationship with the time of thrombus appearance. The values of peak CPK levels and the degree of left ventricular wall motion abnormalities observed in patients with early left ventricular thrombus were significantly higher than the values detected in patients without thrombi, but similar to those obtained in patients with later thrombus occurrence.(ABSTRACT TRUNCATED AT 250 WORDS)