BACKGROUND. To reduce blood consumption in cardiac surgery, aprotinin has been widely used for years. Because aprotinin is metabolized in the kidney, damage of the renal system has been discussed. METHODS. To study these possibly unfavorable effects of aprotinin, a prospective, randomized, placebo-controlled study of 20 patients undergoing aortocoronary bypass operations was performed. A placebo group P was compared with group A, in which patients received high-dose aprotinin according to the "Hammersmith" regimen. Renal function was assessed for 5 postoperative days using sodium dodecyl sulfate gel electrophoresis and quantitative protein analysis of the urine. RESULTS. During and after the operation, temporary renal dysfunction was found in all patients, with a substantial increase of all investigated indices. The alpha 1-microglobulin level in the urine was significantly increased in the aprotinin group for 5 days in comparison with the placebo group, with a maximum on the third postoperative day (64.8 +/- 13.7 versus 21.0 +/- 6.5 mg/L; p < 0.05). Similarly, after sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the bands of proteins filtrated in the renal tubular system were almost tripled in the aprotinin group 5 days postoperatively (5.0 +/- 0.8 versus 2.1 +/- 0.2; p < 0.05). Although urine production was significantly increased in group A (4789 +/- 580 versus 3653 +/- 492 mL/24 h postoperatively; p < 0.05), no relevant changes in serum or urine creatinine levels could be observed in either group. CONCLUSIONS. Patients undergoing aortocoronary bypass operations demonstrate a temporary renal dysfunction. Aprotinin impairs renal function in addition by overloading the tubular reabsorption mechanisms. Patients with normal renal function preoperatively--as were included in this study--are able to compensate for both the perioperative renal dysfunction caused by the extracorporeal circulation and the additional tubular damage due to aprotinin.