SummaryA case history is presented of a 12 year old girl with pulmonary arteriovenous fistula. The diagnosis was based on clinical and radiological signs and on physiological evidence for a large arteriovenous shunting of blood in the lungs. As an unexpected other finding, diffusing capacity was markedly decreased. The largest radiologically apparent fistula, situated in the right lower lobe, was resected; peroperative clamping of pulmonary arterial blood supply to the affected lobe substantiated the presence of an arteriovenous fistula. Following surgery, arteriovenous shunting was found to be only slightly decreased (from 30 % to 25 % of cardiac output). In the resected lobe cavernous angiomata were demonstrated but also multiple capillary teleangiectases and marked thickening of alveolar walls.We suggest that the finding of a decreased diffuing capacity in patients with arteriovenous fistula indicates diffuse pulmonary disease which precludes surgical correction.A review of recent literature on pulmonary arteriovenous fistula is presented.