On the basis of the hypothesis that pulmonary blood flow during cardiopulmonary bypass (CPB) is related to postoperative pulmonary dysfunction, we divided following two groups in 17 patients with congenital cardiac disease; A group: bronchial blood flow (BF) during CPB < 10%, B group: BF during CPB > 25%. Lung specimens taken immediately after CPB were reviewed for ultrastructural changes using semi-quantitative grading of epithelial cell injury, endothelial cell injury, and measuring thickness of basement membrane (BMT) as the index of interstitial edema. And postoperative pulmonary function using AaDO2 were assessed. Ultrastructural grading of epithelial cell injury, BMT, and AaDO2 of A group (BP < 10) were higher than those of B group (BP > 25%). These results indicate that pulmonary blood flow during CPB somewhat may reduce pulmonary impairment for CPB.