An infant with cyanotic congenital heart disease and polysplenia syndrome developed profound cyanosis within months of undergoing bilateral cavopulmonary anastomoses. Intrapulmonary shunting was diagnosed by contrast echocardiography with peripheral venous and selective pulmonary artery injection. Histopathology revealed abnormal, thin-walled vessels within the interstitium of the lung lobule. These vessels have not been reported previously and are likely to be the anatomic site of arteriovenous shunting. This case demonstrates that pulmonary arteriovenous fistulas (PAVFs) may develop rapidly after cavopulmonary anastomosis in young infants. It also illustrates the use of contrast echocardiography for following PAVF progression in these patients.