Cor triatriatum is a congenital anomaly in which the left atrium (LA) is essentially partitioned into two chambers with a small orifice between them. The cephalad chamber receives the pulmonary veins and the caudal chamber is connected to the mitral valve. The concept of "cor triatriatum" as an acquired condition is new. Recently, however, we studied the heart of a 31-year-old opiate addict who had three chambers at the atrial level rather than two. He had developed Staphylococcus aureus endocarditis of the aortic valve resulting in severe aortic regurgitation (AR), causing severe congestive heart failure and associated with hemorrhagic pericardial tamponade. At operation, each of the three aortic valve cusps was virtually destroyed by the active infection. A ring abscess was found at operation and the mouth into it was just caudal to the junction of the left and posterior cusps. The cusps were excised and the mouth into the ring abscess was debrided. A porcine bioprosthesis (23 mm. size) was inserted at the level of the "anulus." The patient died of a narcotic overdose 75 days after an otherwise uneventful clinical postoperative course. A murmur of AR was never present postoperatively and he had received antibiotics for 6 weeks after operation. At necropsy, an opening was found just caudal to the ring of the bioprosthesis that led into a large chamber situated adjacent to both atria. This accessory chamber, which was much larger than either the right atrium or the compressed LA, ended in a blind pouch, the wall of which consisted entirely of dense fibrous tissue. No residual infection was present in the heart. The bioprosthetic cusps were normal and the ring was securely in place.