Idiopathic ventricular fibrillation is defined as cardiac arrest in the absence of structural heart disease and other identifiable causes of ventricular fibrillation. It occurs in 1% to 9% of survivors of out-of-hospital cardiac arrest. The mean age of these patients is 35 to 40 years, and 70% to 75% are male. The pathogenesis is unknown; psychosocial factors may play a role. Baseline clinical characteristics have not been found to identify the 20% to 30% of patients who will have recurrent cardiac arrest. At present, implantation of an automatic defibrillator is the treatment of choice. Two registries have been established to enhance our knowledge of this unusual catastrophic entity.