A four-year-old German shepherd mixed-breed dog was presented with an asymptomatic heart murmur. An atrial septal defect associated with tricuspid and mitral dysplasia (atrioventricular canal defect) was diagnosed. The pulmonary to systemic blood flow ratio was in the range of 2.5:1 (Qp:Qs ratio). Echocardiography and flow pattern across the valves as well as the direction of the shunt allows a better description of the physiopathological consequences of this rare congenital heart disease. Additionally, an Ostium Secundum Type interatrial septum defect was found at necropsy.