ABSTRACT Severe cardiomegaly with an atrial septal defect was discovered during necropsy of a subadult White-tailed Eagle ( Haliaeetus albicilla ) found dead in the wild. A thin membrane composed of fibromuscular tissue separated the left atrium into two chambers, most consistent with that described for cor triatriatum sinister (CTS) in other species. Seventeen months later, necropsy of an adult White-tailed Eagle again revealed CTS. This lesion has not been reported previously in raptors.