Intraoperative high-dose aprotinin during cardiopulmonary bypass was used to investigate if high-risk bleeders could be changed to bleed normally or less as well as see if aprotinin could preserve lung function. Eleven matched controls were compared with eleven aprotinin patients taking warfarin or aspirin preoperatively. The mean (+/- SEM) 12-h and 24-h postoperative amount of bleeding, volume of blood product transfusion and hemoglobin reduction in the aprotinin group were 328 +/- 45 ml, 418 +/- 63 ml, 341 +/- 99 ml and 1.8 +/- 0.5 g% respectively, which were significantly lower than the respective values of 716 +/- 86 ml (P less than 0.01), 1,029 +/- 115 ml (P less than 0.01), 985 +/- 294 ml (P less than 0.05) and 4.1 +/- 0.4 g% (P less than 0.02) in the controls. There was a 65% blood-saving effect by aprotinin in this study. The hypercapnea rate was 45% in the treated patients, and 82% (P less than 0.05) in the controls reflecting better preservation of pulmonary diffusion function which is clinically important following major surgery.