In this prospective, randomized, placebo-controlled, double-blind trial we studied the effects of IV N-acetylcysteine for prevention of renal injury in patients undergoing abdominal aortic surgery. Seventy patients without previously documented renal dysfunction were randomly allocated to receive either N-acetylcysteine (150 mg/kg mixed in 250 mL of 5% dextrose infused in 20 min, followed by an infusion of 150 mg/kg in 250 mL of 5% dextrose over 24 h) or placebo. The infusion was started after the induction of anesthesia. The primary outcome measure was renal injury as measured by the increases in urinary N-acetyl-beta-d-glucosaminidase (NAG)/creatinine ratio (indicator of renal tubular injury) and urinary albumin/creatinine ratio (indicator of glomerular injury). Renal function was assessed by measuring plasma creatinine and serum cystatin C concentrations. The urinary NAG/creatinine ratio increased significantly from baseline to before crossclamp and remained increased on day 5 in both groups. The urinary albumin/creatinine ratio increased significantly from baseline to 6 h after declamping in the N-acetylcysteine group. However, the changes in the NAG/creatinine ratio and the albumin/creatinine ratio were not significantly different between the two groups. Plasma creatinine and serum cystatin C values remained unchanged during the study period in both groups. In conclusion, N-acetylcysteine did not offer any significant protection from renal injury during elective aortic operation in patients with normal preoperative renal function, and some degree of tubular injury seems to occur before aortic crossclamp.