An eleven-year-old female Pinscher was attended at the Veterinary Hospital of Federal University of Minas Gerais with a history of anorexia and depression. On physical examination, reduction of heart sounds, jugular pulse, tachypnea and peritoneal fluid were evidenced. Survey thoracic radiographic revealed pleural effusion and right atrium enlargement. The echocardiography demonstrated a large interatrial communication with 0.7cm at the right parasternal long axis view and colour Doppler allowed the confirmation of a left-to-right shunt. In addition, a continuous flow was observed on spectral Doppler echocardiography at the defect region (peak velocity, 4.53m/s; pressure gradient, 82.08mmHg). There was increase in the systolic function because the fractional shortening was high (54%). Moreover, the transthoracic echocardiography revealed discretely thickened atrioventricular valve leaflets and Doppler study demonstrated mild mitral insufficiency and severe tricuspid regurgitation. Enlargement of the right atrium, right ventricle and main pulmonary artery were detected. The most of the routine hematological studies were normal. The enzymes alkaline phosphatase and alanine aminotransferase were elevated. The ultrasound revealed ascites and congestive hepatomegaly. Laboratory analyses of peritoneal fluid revealed simple transudate. The treatment consisted of oxigenotherapy, drainage of peritoneal fluid, administration of furosemide and hydrochlorothiazide however, clinical improvement was not observed. Necropsy demonstrated a secondary type of interatrial communication with the size of 0.8cm, pleural effusion, hepatic congestion and peritoneal fluid. Atrial septal defect (ASD) was considered a rare disease however, with a better definition of echocardiography, the diagnosis has increased. This disease is characterized by communication between the two atrium. Isolated ASDs usually result in a left-to right shunting across the defect because the left atrium normally works with greater pressure. ASD is usually a well-compensated disease and clinical signs are related to a concurrent heart disease. In this case, it 's believed that the onset of clinical signs in elderly dog was due to erratic flow secondary to the increase of mitral degeneration. Therefore, the ASD can cause clinical signs in adult animals and the echocardiography combined with colour-flow Doppler mode was the method that is chosen to confirm ASD.