The clinical course in 15 patients with features consistent with arrhythmogenic right ventricular dysplasia is described. At referral seven patients had abnormal physical findings, nine had abnormal electrocardiograms with non-specific right-sided abnormalities, and seven patients had increased heart size or prominent right ventricles on chest x ray. During long term follow up (mean 8.8 years, range 1.5 to 28 years) 11 patients had abnormal physical findings, 11 had electrocardiographic changes, and nine had increased heart size. Recurrent sustained right ventricular tachycardia was the most common arrhythmia (10 patients). Two patients experienced ventricular fibrillation. Seven patients suffered from over 10 episodes of ventricular tachycardia, nine required cardioversions, and 10 patients had associated serious symptoms such as syncope, severe hypotension, or cardiac arrest. Four patients required operation to correct the arrhythmia and three patients developed right heart failure. Two out of three deaths were sudden. These data suggest that in arrhythmogenic right ventricular dysplasia right ventricular abnormalities may be progressive and that the condition may affect the left ventricle. The course of the ventricular arrhythmias was highly variable and could not be predicted in individual patients. The potential for lethal ventricular arrhythmias is evident and warrants intensive diagnostic efforts to identify patients with adverse prognostic features.