An 11-month-old, female Yorkshire terrier was referred with a 1-month history of hyporexia, depression and exercise intolerance. On a physical examination a continuous grade VI/VI murmur was best heard at the left heart base and fine crackles were evidenced over the hilar region. In six lead electrocardiogram (EKG) the R wave had a voltage of 2.7 mV and the QRS duration 0.08 ms, suggesting left ventricular (LV) enlargement. In the survey thoracic radiographs, the left atrial (LA), right atrial (RA), LV and the main pulmonary arterial segment enlargement were evident. There was hilar alveolar pattern in lateral view. Transthoracic echocardiography was performed which revealed aortopulmonary septal defect (APSD) and the results of colour-flow and spectral Doppler echocardiography were indicative of a continuous turbulent flow in the main pulmonary artery. There were left atrial enlargement (left atrium/root aortic ratio in two-dimensional, 2.5) and severe left ventricular dilatation in M-mode (28.5 mm), the fractional shortening was high (FS, 54%) showing an increase in the systolic function. Routine hematological and biochemical studies were normal. The dog serum sample had positive antibody titers against Leishmania sp. therefore euthanasia was performed and at the necropsy the APSD was confirmed. The APSD is characterized by a communication between the ascending aorta and the pulmonary artery in the presence of separate semilunar valves. In dogs, clinical signs due to APSD have only been reported four times previously, all of them were females. This disease shares the same pathophysiology as patent ductus arteriosus (PDA), which represents one of the most common congenital cardiovascular defects in the dog. In the present case, some clinical findings were consistent with PDA because of the detection of a high continuous murmur. On EKG and in the survey thoracic radiography were suggestive of PDA however, RA enlargement is not a typical radiographic finding of PDA. The APSD is more severe than PDA because the opening is generally larger, the defect is a more direct shunt and, at the present moment, there has never been successful correction surgery performed in dogs. APSD should always be considered when the left atrium is marked dilated or there is severe pulmonary hypertension.