The term “idiopathic” ventricular fibrillation is used to describe those episodes of unexpected sudden arrhythmic death due to ventricular fibrillation in patients with no demonstrable structural heart disease. Idiopathic ventricular fibrillation has been reported to account for 5–10% of all sudden arrhythmic deaths. Post mortem analysis have shown that about 80% of patients might have some kind of structural anomalies, mainly atherosclerosis, myocarditis, or right ventricular dysplasia. Follow-up of patients with idiopathic ventricular fibrillation has shown a high incidence of recurrent episodes of malignant ventricular arrhythmias. The absence of structural heart disease generally implies an excellent long-term prognosis if ventricular fibrillation can be avoided. Patients with an implantable defibrillator should have a mortality rate similar to the general population. New subsets of patients are being recognized as belonging with those previously classified as idiopathic ventricular fibrillation. More than 60 patients have been identified in different centers around the world with the so-called “right bundle branch block, ST segment elevation, and sudden death syndrome.” Recurrence rate of malignant ventricular arrhythmias is very high in these patients, despite antiarrhythmic therapy. An implantable cardioverter-defibrillator seems the treatment of choice. Asymptomatic forms of the syndrome have been described. Follow-up in these asymptomatic patients has shown that some of them might become symptomatic during follow-up. Also, intermittent forms of the syndrome have been described, with transient normalization of the electrocardiogram. Administration of class I drugs in these patients unmasks the typical electrocardiographic pattern. In some of the patients previously classified as having idiopathic ventricular fibrillation, ajmaline or procainamide administration unmasks the electrocardiographic pattern of the syndrome, suggesting that its incidence may be higher than previously suspected. (Am J Cardiol 1996;78(suppl 5A):69–75)