A case of congenital communication between the right pulmonary artery and the left atrium is presented and the previous literature relative to this reviewed. Clinically, this condition differs only slightly from the more common pulmonary arteriovenous fistula. Physiologically, the pulmonary vascular resistance is increased at rest, but responds with normal lowering of resistance on exercise and breathing of 100 per cent oxygen. The fistula resistance, as expected, remained constant, resulting in increased pulmonary blood flow during exercise and breathing of 100 per cent oxygen, a decrease in the percentage of right to left shunt and an increase in arterial oxygen saturation in these physiologic conditions. It would seem, at least in this case, that the increase in the pulmonary vascular resistance is probably reflex in origin due to hypoxemia rather than to organic occlusion of pulmonary vessels by multiple pulmonary thrombi.