Interatrial abscess.

A large abscess of the interatrial septum developed during the course of acute bacterial endocarditis affecting the aortic valve. Septic involvement of the cardiac conducting system produced atrioventricular dissociation, and subsequent abscess rupture resulted in an aorto-right atrial communication. The clinical presentation of this rare complication of acute bacterial endocarditis is correlated with the post-mortem findings.