The cor triatriatum sinistrum is a rare congenital malformation. Frequ
ently, the patients become symptomatic in infancy with clinical signs
of severe congestive heart failure. We report on an adult patient with
only mild symptoms with cor triatriatum sinistrum and atrial septal d
efect. The embryological basis of the disorder and the hemodynamic con
ditions which permitted this patient to remain symptom free for a long
period are discussed.