Mature cardiac myocyte hamartoma in the right atrium.

During coronary artery bypass grafting in a 58-year-old man, a mass was discovered incidentally in the right atrium, measuring 1.5 x 1 x 0.5 cm. It was composed of disorganized hypertrophic mature cardiac myocytes, and associated with focal fibrosis, mature adipocytes, and mild lymphocytic infiltration in peripheral areas, indicative of cardiac hamartoma. This type of hamartoma has been rarely reported as an isolated mass in the right atrium.