We examined the causes of death in patients previously resuscitated from out-of-hospital ventricular fibrillation. In 51 months, 234 patients were sucessfully resuscitated, hospitalized and discharged home. During follow-up observation, 89 episodes of recurrent ventricular fibrillation or death (or both) occurred, 64 of which (72 per cent) were unexpected, out-of-hospital circulatory arrests. Ventricular fibrillation occurred in 77 per cent of the 44 episodes in which the electrocardiogram was observed. Ten patients survived one or more episodes of recurrent ventricular fibrillation. Median time to recurrent ciruclatory arrest was 20 weeks after the preceding episode. Prodromal symptoms were infrequent, and activity levels were generally low at the time of cardiac arrest. Recurrent ventricular fibrillation of sudden death (or both) occurred predominantly when the initial episode was not associated with acute myocardial infarction. Patients resuscitated from ventricular fibrillation are susceptible to early recurrence, probably reflecting continuing myocardial electrical instability.