Background: N-acetylcysteine and ascorbic acid have been reported to be efficacious in preventing contrast induced nephropathy (CIN) after coronary angiography through their antioxidants property. However there have been many controversies regarding their role. We evaluated the efficacy of widely used antioxidant- alpha lipoic acid (ALA) for prevention of CIN. Methods: We conducted a prospective randomized controlled trial to evaluate the efficacy of ALA in CIN prevention. A total of 108 patients with basal renal insufficiency (creatinine clearance [CrCl] <60ml/min) were randomized to have ALA for 2 days (600mg orally three times a day before and after coronary catheterization, totally 3600mg, n=53) or placebo (n=55). All the patients received hydration with iso-tonic saline at a rate of 1mL/kg/hour at least for 12hours before and 12hours after administration of contrast agent. Only iso-osmolar contrast medium - iodixanol was used in the procedure. The primary end point was the peak increase of SCr and the secondary end point was the incidence of CIN within two days. CIN was defined as an increase in SCr of either ��� 25% or ��� 0.5 mg/dL. Results: There were no differences in age, sex, prevalence of diabetes, prevalence of hypertension, baseline SCr, baseline left ventricular systolic function, and contrast media dose. The mean peak increase of SCr was significantly higher in ALA group than placebo: 0.004짹0.37mg/dL vs. -0.115짹0.186mg/mL in ALA and placebo respectively (p=0.037). The SCr did not change from baseline of 1.24mg/dL to 1.24mg/dL within 2 days in the ALA group (p=0.941), but decreased significantly from 1.16mg/dL to 1.04mg/dL in the simple iso-tonic saline hydration group (p<0.001). The incidence of CIN was similar in both groups, 3.8% vs. 3.6% for ALA and placebo group (p=1.0). Conclusion: We found for the first time that alpha lipoic acid – a well known antioxidant may be inferior to simple hydration with iso-tonic saline in CIN prevention. This study may imply that not all antioxidants are beneficial for preventing CIN in patients undergoing coronary angiography.