Arrhythmogenic right ventricular dysplasia/cardiomyopathy (ARVC) is characterized histologically by massive infiltration of the right ventricular wall by fat tissue, with surviving strands of cardiomyocytes bordered by or embedded in fibrosis. ARVC has been recognized as a cause of sudden death, especially in the young. The purpose of our autopsy study was to examine the clinical characteristics and the pathological patterns in the hearts of 20 people who died suddenly of ARVC. In view of our findings and the literature, we discussed the possible causes and pathogenesis of ARVC, as well as the mechanisms by which sudden death occurs in this disease. During the 7-year study period, 20 hearts from 9 men and 11 women fulfilled the criteria for ARVC. The mean age was 41 years (range, 17 to 80). The disease was unknown prior to death in all cases. Fourteen persons died at rest, and six on effort. In 9 of the 20 cases, the trigger of sudden death was an acute emotional stress, sometimes associated with a moderate physical activity. The mean heart weight was 380 g (range, 280 to 520). Both ventricles were involved in 40% of the cases. Inflammatory infiltrates consisting of lymphocytes were present in 60% of the cases, but myocyte necrosis was found in only one case. ARVC is more likely to result from a degenerative process than a congenital disorder. Genetic factors, viral or autoimmune inflammation or both, and apoptosis are also involved in the degenerative disorder.