A family with the heritable syndrome of prolonged Q-T interval is reported. Three members died suddenly. Six had a prolonged Q-T interval. One had syncopal episodes for four years but has had no syncope since therapy with propranolol was initiated two years ago. The inheritance appears to be autosomal dominant. There was no evidence of hearing defects. Histologic and electron microscopic studies were performed on cardiac tissue from one patient. This is the first report of electron microscopic studies, and the findings suggest a possible defect of calcium metabolism in the myofiber.