BACKGROUND It has been shown that N-acetylcysteine may be useful in correcting postoperative hepatic and renal function in many pathological conditions. The present study aimed to examine the effect of N-acetylcysteine on liver and kidney function tests after surgical bypass in patients with obstructive jaundice. METHODS & Materials: A total of 30 patients with obstructive jaundice who were candidates for bypass surgery were enrolled in this randomized clinical trial. In the case group, intravenous N-acetylcysteine (200 mg/kg per hour in the first 8 h, followed by 100 mg/kg per hour for another 16 h, the same dose for another 24 h) was administered postoperatively. Liver and renal function tests (serum AST, ALT, ALP, GGT, bilirubin, and creatinine) were compared between two groups, as well as duration of hospitalization and ICU stay. RESULTS Postoperatively, decrease in mean serum AST (p = 0.01), ALT (p = 0.02), ALP (p = 0.01), GGT (p = 0.04) and bilirubin (total, p = 0.02, direct, p = 0.01) levels compared to the preoperative values was significantly more among cases compared to those in controls. Changes in serum creatinine, however, did not differ significantly between two groups (p = 0.18). Hospital and ICU stays were also not different between two study groups (p = 0.27 and p = 0.94 respectively). CONCLUSIONS On the basis of our findings, intravenous N-acetylcysteine in patients with obstructive jaundice could significantly preserve liver function after bypass surgery. Effect of this medication on renal function; however, was not statistically significant. BACKGROUND Iranian Registry of Clinical Trial: IRCT2016041016473N7.