In 5,839 consecutive patients with acute myocardial infarction (AMI), hospitalized between July 1981 and July 1983 in 14 coronary care units in Israel, the incidence of primary ventricular fibrillation (VF) was 2.1%. Patients with primary VF resembled counterparts without VF in terms of age, gender, frequency of previous AMI and past cigarette smoking habits. The hospital course of patients with primary VF revealed increased incidence of primary atrial fibrillation and atrioventricular block. Increased serum levels of glutamic oxaloacetic transaminase and lactic dehydrogenase were noted among the patients with primary VF. In-hospital mortality rate was 18.8% in 122 patients with primary VF compared with 8.5% in 3,707 patients forming the reference group (p less than 0.01). Adjustment by age using logistic function yielded an estimate of 2.86 for relative mortality odds associated with primary VF, and further adjustment by gender, history of AMI, systemic hypertension, and by enzymatically estimated infarct size slightly reduced the estimated odds, at 2.52 (95% confidence interval, 1.42 to 4.46). Prognosis after discharge from the hospital was independent of primary VF. In conclusion, primary VF exerts an independent, significant effect on in-hospital mortality.