To explore the impact of plasma homocysteinemia(Hcy) on contrast-induced nephropathy (CIN) after percutaneous coronary intervention (PCI) in acute coronary syndrome (ACS) patients.Consecutive 684 ACS patients undergoing first PCI in our department between January 2013 and December 2014 were prospectively enrolled.Patients were divided into 2 groups according to the pre-procedural plasma Hcy level: high-Hcy group (Hcy≥10 μmol/L, n=404) and control group (Hcy10 μmol/L, n=280). The CIN was defined as serum creatinine ≥ 44.2 μmol/L or 25% increase compared to baseline within 48-72 h after PCI.The baseline clinical data and the ratio of CIN were compared between the 2 groups.Multivariate logistic regression analysis was used to define the independent risk factors for CIN.CIN occurred in 133(19.4%) out of 684 enrolled patients, and the incidence of CIN was significantly higher in high Hcy group than in the control group (22.0%(89/404)vs. 15.7%(44/280), P=0.040). After adjusting the confounding factors, including age, acute myocardial infarction, co-morbidities(hypertension, diabetes mellitus, and old myocardial infarction), laboratory examination (level of cystatin C and uric acid), glomerular filtration rate, left ventricular ejection fraction, angiographic and procedural characteristics (3 diseased vessels, multiple stent implantation), treatment at admission (spironolactone, digoxin), multivariate logistic regression analysis showed that high Hcy was independently associated with the development of CIN (OR=1.70, 95%CI 1.60-2.64, P=0.021).Elevated Hcy prior PCI is an independent risk factor of CIN in ACS patients undergoing first PCI.