The use of aprotinin to reduce blood loss after cardiopulmonary bypass is under debate. Concern has been raised about the renal effects of aprotinin. We administered a mean aprotinin dose of 4.2 x 10(6) kallikrein-inhibiting units to 13 patients with coronary disease undergoing cardiopulmonary bypass for 74 +/- 5 minutes (mean +/- standard error of the mean); 13 comparable patients having cardiopulmonary bypass served as control subjects, and all were studied postoperatively for 24 hours. Aprotinin reduced postoperative blood loss by 50% (p = 0.0082). Two of the 13 patients who received aprotinin needed one red cell unit each versus a total of 18 units in eight of 13 control patients (p = 0.0096). Blood pressure, hemoglobin value and serum protein concentration were higher after operation in the aprotinin group (p less than 0.05 to p less than 0.01). Platelet counts did not differ, but plasma thromboxane was lower in aprotinin recipients (p less than 0.001). In control patients fibrinogen degradation products (D dimer) doubled, and alpha 2-antiplasmin activity was halved during and after cardiopulmonary bypass (p less than 0.01 to p less than 0.001), whereas aprotinin patients showed no changes. The complement breakdown products C4a, C3a, and C3dg as well as C9 neoantigen increased from prebypass baseline in both groups (p less than 0.001); the increment of C3a and C3dg was greater in the aprotinin than in the control patients (p less than 0.001). Serum electrolytes, osmolality, and creatinine remained normal in both groups of patients. Creatinine clearance was normal or above normal and virtually identical in both groups. Osmolar clearance and fractional sodium excretion were higher in the aprotinin group than in the control group shortly after cardiopulmonary bypass (p less than 0.05 to p less than 0.01); renal function was unremarkable the next morning. No adverse clinical effects attributable to aprotinin were seen. In summary, aprotinin offers advantages for cardiopulmonary bypass.