The urinary excretion of N-acetyl-beta-D-glucosaminidase (NAG) was evaluated in terms of its ability to predict renal failure following open-heart surgery. Twenty-two patients were divided into three groups; 6 patients with post-operative renal failure (RF group), 9 without renal failure (non-RF group) and 7 who underwent non-cardiac surgery (non-ECC group). NAG levels during operation were significantly higher in the non-RF group than in the non-ECC group (2.28 +/- 0.50 U vs. 0.39 +/- 0.06 U, p < 0.01). NAG levels in the RF group were not significantly different from those in the non-ECC group (1.12 +/- 0.28 U/hr vs. 0.39 +/- 0.06 U/hr, p > 0.1), and in the non-RF group (1.12 +/- 0.28 U/hr vs. 2.28 +/- 0.50 U/hr, p > 0.1). Post-operative NAG levels were not significantly different between the RF and non-RF groups, whereas post-operative creatinine clearance values were significantly different (on 2, 6 and 7 post-operative day, p < 0.05). Our data suggest that the kidney is predisposed to renal failure by cardiopulmonary bypass and the chance of renal failure depends in some part on post-operative patient management.