Introduction Congenital diverticulum of the atrium is an extremely rare deformity. This is the report of a congenital left atrial diverticulum excised by Dr. Willis J. Potts from a 2½-year-old boy in 1954. Report of Case A 2½-year-old boy referred by Dr. Mark F. Canmann was first admitted to The Children's Memorial Hospital, Chicago, on Nov. 17, 1954. Chief Complaint. —"Cough, chest tumor found on x-ray, and rapid pulse." Present Illness. —The child was apparently well, although somewhat weak and thin, until 4 to 5 weeks before admission, when he developed an intermittent cough, followed on occasion by vomiting. He was febrile, with temperature rises to 38.3 C and 38.8 C. Scattered rales and rhonchi were heard throughout his chest. Two weeks after the onset of the illness he was admitted to the High-land Park Hospital, Highland Park, Ill. He responded in 48 to 72 hours with rest and antibiotics,