Subdivided left atrium mimicking a cardiac tumor.

We describe a 25-year-old man with a subdivided left atrium. The lesion was misdiagnosed preoperatively as a cardiac tumor because echocardiographic and magnetic resonance imaging revealed a solid mass arising from the posterior wall of the left atrium. Cardiac surgery revealed a small accessory chamber draining the two left pulmonary veins. No membranous structure was evident between the chamber and the left atrium. The solid mass identified noninvasively was a hypertrophic muscle which formed a wall of the accessory chamber.