Pulmonary arteriovenous fistula is a congenital malformation resulting from errant capillary development, with incomplete formation or disintegration of the vascular septa that normally divide the primitive connections between the venous and arterial plexuses. It generally occurs as part of the disorder known as hereditary hemorrhagic telangiectasia [Rendu-Osler-Weber disease]. The hereditary lesion is transmitted as a simple non-sex-linked dominant trait. It may be single or multiple, too small to see on plain chest films or large and easily recognized. One third of the lesions are multiple on plain chest film. The pathogenesis of its symptoms is that unoxygenated, desaturated arterial blood enters into the pulmonary venous system, directly. Recently we have experienced a case of the pulmonary arteriovenous fistula in 26 years old male soldier, which was confirmed by pulmonary angiography preoperatively. 2 thumb-tip sized, well circumscribed cystic masses filled with bright red colored blood were seen in subpleural and anterolateral portion of the right upper lobe. Right upper Iobectomy was performed due to close approximation of the fistula with pulmonary vein. Microscopically, it shows angiomatous dilatation of the abnormal vessels embedding in the parenchyma. Postoperative physiologic studies show nearly normal arterial oxygen saturation, hemoglobin and RBC count. There was good, uneventful postoperative course. [KTCS 1983;3:362-367]