Background: Contrast induced nephropathy (CIN) is a leading cause of hospital acquired renal failure and reported to affect the mortality and morbidity. N-acetylcysteine and ascorbic acid have recently been reported to prevent CIN in renal insufficiency by its antioxidant property. There has been no study comparing the efficacy for CIN between the two antioxidants. Methods: We conducted a prospective randomized controlled trial to compare two antioxidants. A total of 212 patients who have basal renal insufficiency of creatinine clearance (CrCl) <60ml/min and/or serum creatinine (SCr) ��� 1.1mg/dl were randomized to have high-dose N-acetylcysteine (h-NAC) (1200mg orally twice a day before and on the day of coronary catheterization, n=106) or ascorbic acid (AA) (3g and 2g orally before, and 2g twice after coronary catheterization with 12hour interval, n=106). Half-isotonic saline was given intravenously at a rate of 1ml/kg/hour at least for 12hours before and 12hours after administration of contrast agent. Only one contrast medium- iodixanol was used in coronary catheterization. SCr was measured on the day 1 and day 2 after coronary catheterization. The primary end point was peak increase of SCr and the secondary end point was incidence of CIN within two days. CIN was defined as increase in SCr of either ��� 25% or ��� 0.5 mg/dL. Results: There were no differences in demographic and procedural characteristics. The mean peak increase of SCr was significantly lower in h-NAC group: -0.029짹0.18mg/dL in h-NAC group vs.0.036짹0.20mg/mL in AA group (p=0.029). The serum creatinine concentration decreased from 1.44mg/dL to 1.42mg/dL in h-NAC group but did not reach the statistical significance (p=0.157) and it increased from 1.29mg/dL to 1.33mg/dL in AA group with no significance (p=0.093). In the secondary endpoint of incidence of CIN, it is less in h-NAC group than AA group: 1.2% in h-NAC and 4.4% in AA group but attained no statistical significance (p=0.37). Conclusion: In our first comparison of two antioxidants for prevention of CIN, the high-dose N-acetylcysteine may be more beneficial than ascorbic acid in renal insufficiency undergoing coronary angiography.