Whether ventricular fibrillation occurring within 48 h after acute myocardial infarction is associated with particular clinical features and poor prognosis, especially in patients with anterior myocardial infarction, is still debated. Therefore, clinical variables and in-hospital and 1 year mortality rates were analyzed in 2,088 patients, aged 18 to 95 years (mean +/- SD 64 +/- 12), admitted to the hospital with acute myocardial infarction between 1979 and mid 1984. One hundred forty-seven patients (7%) had at least one episode of ventricular fibrillation occurring within 48 h of hospital admission. Of these, 25% died during their initial hospitalization compared with 13% of patients without early ventricular fibrillation (p less than 0.001). In greater than 50% of patients with early ventricular fibrillation, the immediate cause of death was left ventricular failure or cardiogenic shock. In contrast, the 1 year mortality rate after hospital discharge was not significantly greater in patients with than in those without early ventricular fibrillation (15 versus 11%, respectively), particularly in the subgroup of patients with anterior myocardial infarction in which the mortality rate tended to be lower in patients with early ventricular fibrillation (8 versus 14%, respectively). Similar mortality results were found when only primary (not associated with left ventricular failure) ventricular fibrillation was analyzed. The left ventricular ejection fraction and the incidence of complex ventricular arrhythmias from 24 h ambulatory electrocardiographic monitoring obtained at hospital discharge were not different in survivors with or without early ventricular fibrillation (0.45 +/- 0.13 versus 0.49 +/- 0.14 and 41 versus 41%, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)