Background: Pharmacokinetic data suggests that the intravenous form of n-acetylcysteine (NAC) may be more effective than the oral formulation in preventing contrast induced nephropathy (CIN). NAC owing to its anti-oxidant properties might be beneficial for patients with acute coronary syndromes (ACS) who are at increased risk for CIN. The aim of this prospective randomized, single-center, double-blind, placebo controlled trial (NCT00939913) was to assess the effect of high-dose intravenous NAC on CIN in ACS patients undergoing coronary angiography and/or percutaneous coronary intervention (PCI). Methods: We randomized 398 ACS patients scheduled for diagnostic angiography ± PCI to an intravenous regimen of high-dose NAC (1,200 mg bolus followed by 200 mg/hr for 24 hr; n = 206) or placebo (n = 192). The primary end-point was incidence of CIN defined as an increase in serum creatinine concentration ≥25% above the baseline level within 72 hr of the administration of intravenous contrast. Results: There was no difference found for the primary end point with CIN in 16% of the NAC group and in 13% of the placebo group (p = 0.40). Change in serum cystatin-C, a sensitive marker for renal function, was 0.046 ± 0.204 in the NAC group and 0.002 ± 0.260 in the control group (p = 0.07). Conclusion: In ACS patients undergoing angiography ± PCI, high-dose intravenous NAC failed to reduce the incidence of CIN. © 2011 Wiley Periodicals, Inc.