A case of pulmonary arterio-venous microfistula in a fifteen year old male is reported. He referred dyspnea and cyanosis for 3 and 2 years respectively. Physical examination was normal except for finger clubbing. Chest radiograph and EKG were also normal. Lung function tests showed hypoxemia due to an increase in veno-arterial shunt (Qs/Qt). Its localization was not apparent by routine studies such as phonocardiogram, echocardiogram, right heart catheterization and pulmonary angiogram. However, a contrast echocardiogram and selective occlusion of the right and left pulmonary arteries performed with a Dotter-Lucas catheter were suggestive of an intrapulmonary localization of the shunt. A lung biopsy confirmed the diagnosis.