Translocation of the atrial septum to the left of the mitral valve is a congenital anomaly: the author has not found a similar case previously reported in the English literature. The curious anomaly described here was associated with deformed mitral and tricuspid valves, a postero-inferior atrial septal defect, a persistent left superior vena cava, a hypoplastic aorta, and a short innominate artery. The patient lived to a remarkable age.