Two hundred sixteen patients undergoing coronary artery bypass graft procedures were randomized to receive either high-dose aprotinin or placebo. Clinically important postoperative renal insufficiency was infrequent, with a single patient (0.9%) from each group requiring dialysis. Although increases in the serum creatinine level occurred postoperatively in more patients who received aprotinin (20/108) than in those given placebo (13/108), the difference between the two groups was not statistically significant (p = 0.186), and the increases were generally small and transient. Likewise, there was no difference between the groups in terms of the incidence of abnormal serum electrolyte levels, blood urea nitrogen levels, or urinalysis findings, or in the frequency of abnormal creatinine clearance rates. Under the conditions described, aprotinin use does not appear to be associated with a significant risk of serious renal toxicity.