We describe an adult patient, in whom three-dimensional transesophageal echocardiography was able to make a definitive diagnosis of cor triatriatum associated with common atrium. The lesion mimicked a classical partial atrio-ventricular septal defect on both transthoracic and transesophageal two-dimensional echocardiography. We found only two cases in the literature describing the existence of cor triatriatum with common atrium, none diagnosed by echocardiography.