Contrast-induced nephropathy (CIN) is a common complication of cardiac catheterization, reported to result in a 15% incidence of acute renal failure. Convincing evidence supports the prophylactic use of prehydration and low volumes of contrast medium. Recently, the antioxidant acetylcysteine has been shown to have a potential preventive role. The aim of this study was to examine the hypothesis that acetylcysteine prevents CIN. Patients undergoing cardiac catheterization with a serum creatinine ≥ 1.5 mg/dl were prospectively randomized to receive acetylcysteine or placebo. A total of five doses of acetylcysteine 600 mg b.i.d. or placebo was administered, commencing on the day of the procedure. All patients were prehydrated with 0.45% saline and during the catheterization a nonionic low-osmolality contrast medium was used. Serum creatinine and urea were measured at 24, 48, and 72 hr postprocedure. A total of 43 patients were studied. There was no significant difference between the groups in terms of baseline characteristics, including baseline renal function. No adverse events were experienced with acetylcysteine treatment. Serum creatinine levels at 48 and 72 hr remained largely unchanged in the acetylcysteine group but continued to rise at 48 and 72 hr in the placebo group. By 72 hr, the incidence of CIN, defined as a 25% increase in baseline creatinine, was significantly lower in the acetylcysteine arm compared to placebo (5% for acetylcysteine vs. 32% for placebo; P = 0.046). In patients with mild to moderate renal impairment undergoing cardiac catheterization, prophylactic treatment with oral acetylcysteine reduces the incidence of contrast-induced nephropathy. Catheter Cardiovasc Interv 2003;60:458–461. © 2003 Wiley-Liss, Inc.