Heterotopic epithelial replacement (so-called “mesothelioma”) of the atrioventricular node is a rare cause of congenital heart block and sudden death; only about 50 cases have been reported in the literature in predominantly female patients ranging in age from infancy to 86 years. The lesion is virtually impossible to diagnose clinically and can be recognized at postmortem only by histologic examination of the atrioventricular conduction tissue. A unique case is reported since the patient, a young, asymptomatic woman, was chosen by chance as the subject of an electrophysiologic study of bradycardia 6 years before her sudden death. The morphologic (light-and electron-microscopic) findings of the atrioventricular node studied at autopsy were supportive of heterotopic epithelial origin of the lesion. The disputed histogenesis is briefly discussed.