A three-month-old puppy had ascites, but its heart was normal by auscultation. Abdominal ultrasonography revealed an enlarged liver, distended hepatic veins and a distended caudal vena cava. Doppler ultrasonography detected retrograde flow in the caudal vena cava and abnormally pulsatile flow in the hepatic veins and caudal vena cava. A non-selective venogram was used to detect the path of the blood from the caudal vena cava. A postmortem examination showed that the puppy had cor triatriatum dexter and a defect in the atrial septum.