Pulmonary arteriovenous fistula is a congenital vascular malformation In the lung, various synonyms including Pulmonary cavernous vascular malformation, Pulmonary arteriovenous aneurysm, Cavernous hemangioma of the lung0 Pulmonary telangiectasia, Pulmonary hamartoma, etc. The pathogenesis of its symptoms is that unoxygenated, desaturated arterial blood enters into the pulmonary venous system directly. Recently we have experienced one case of the pulmonary arteriovenous fistula which was diagnosed as the pulmonary cystic lesion of the lung preoperatively in 20 years old, 61 kg, male patient. Operation was revealed well circumscribed cystic lesion filled with blood, subpleural and anterior mediobasal location, and bright red colored aspirates on two times needle aspirations. Microscopic finding shows ill circumscribed vascular lesion composed of varying sized blood vessels with irregular thickening of wall and final pathological diagnosis is Pulmonary Arteriovenous Fistula. Basal segmentectomy was done and the patient shows good postoperative course. [KTCS 1982;2:183-187]