Introduction: PDA represents one of the most frequently diagnosed type of congenital heart disease. Ductus arteriosus is a normal structure in foetal life, which permits shunting of oxygenated blood from the pulmonary artery into the aorta. Failure of sealing after birth is an abnormal condition and is called patent ductus arteriosus. In normal PDA, due to fact that systemic pressure is fivefold higher than pulmonary circulation, blood is shunted from the aorta into the pulmonary artery. In reverse PDA, pulmonary artery pressure does not drop after birth, and blood will be shunted form right to left. Aims: We want to evaluate clinical, haematological, ECG and echocardiographic changes in case of reverse PDA. Materials and Methods: Two-year old female Bichon Frise was referred to our clinic with signs of effort intolerance and dyspnoea for more than a year. ECG was performed in the right lateral recumbency using a digital device and echocardiography was done with Esaote MyLab40 Vet with a phased array transducer matched with the size of the dog (7.5 MHz). Results: We identified a dog with a good body score, quite alert and without any sign of illness. Haematological investigation underlined polycythaemia and very high PCV. The ECG revealed a normal sinus rhythm with a deep S wave, changes consistent with right ventricle enlargement.  Right atrial dilation and right ventricle hypertrophy were found on cardiac ultrasonography. The right ventricle free wall was hypertrophied and interventricular septum was flattened, changes consistent with increased pressure on the right side of the heart. The left heart was small. Positive diagnosis was done, performing “bubble study” and identification of contrast bubble within the abdominal aorta.   Conclusion: Reverse PDA is a rarely diagnosed congenital heart disease. Polycythaemia in young dogs could raise the suspicion of reverse PDA.  For positive diagnosis, echocardiography and bubble study are required. ECG is not a sensitive tool for diagnosis.