BACKGROUND As part of a population-based study of acute myocardial infarction, we examined changes over time in the incidence and in-hospital case-fatality rates of primary ventricular fibrillation complicating acute myocardial infarction. RESULTS Patients with validated acute myocardial infarction hospitalized at 16 hospitals in the Worcester, Mass, metropolitan area between 1975 and 1990 comprised the study sample. During the 15-year study period, 5.1% of patients developed primary ventricular fibrillation in the setting of uncomplicated acute myocardial infarction, with this rate remaining relatively constant over time. Both age- and multivariable-adjusted analyses showed no significant trend in the incidence rates of primary ventricular fibrillation during the study period. The in-hospital case-fatality rate for patients with primary ventricular fibrillation was significantly elevated compared with the rate for those without primary ventricular fibrillation and uncomplicated acute myocardial infarction (48.3% versus 1.5%, P < .001). No significant change over time was noted in in-hospital case-fatality rates associated with primary ventricular fibrillation while controlling for a variety of short-term prognostic factors. CONCLUSIONS The results of this communitywide observational study suggest that neither the incidence nor the prognosis associated with primary ventricular fibrillation resulting from acute myocardial infarction has improved over time.