Classic cor triatriatum is rarely found in adults. Preoperative assessment of classic cor triatriatum in a 22 year old man without symptoms by three dimensional echocardiography was more informative than transoesophageal echocardiography or magnetic resonance imaging, which both showed only a small hole in the membrane separating the accessory atrium from the true left atrium. The size of the hole indicated a strong likelihood that symptoms would develop in this patient. Dynamic three dimensional echocardiography, however, showed a long wide hole in the separating membrane. This finding was consistent with the absence of symptoms in this patient. Symptom free patients with moderate obstruction do not need early surgical correction.