Serum YKL-40, a potential inflammatory marker, is greatly increased at the early stage of ST-segment elevation myocardial infarction (STEMI). Here, we hypothesized that YKL-40 levels at admission could predict the long-term outcomes after STEMI. A total of 324 patients with acute STEMI undergoing primary percutaneous coronary intervention (PCI) were consecutively enrolled and followed for 24 months. The baseline clinical and procedural data were recorded, and serum YKL-40 levels at admission were measured using ELISA method. The endpoint of interest was major adverse cardiac event (MACE), including all-cause death, recurrent myocardial infarction, and hospitalization for heart failure. Patients with elevated serum YKL-40 levels (>= 126.8 ng/mL) were more likely to be older and smoker and to present with type 2 diabetes, advanced Killip class, multivessel disease and intra-aortic balloon pump, with increased levels of admission glucose, triglyceride, and high-sensitivity C-reactive protein and decreased level of high-density lipoprotein cholesterol. During the follow-up period, the incidence of MACE was notably higher in the high than in the low YKL-40 groups (28.4% vs 11.1%, P<.001). Kaplan-Meier curve showed that elevated YKL-40 levels were associated with reduced MACE-free survivals (log-rank P<.001). In multivariate Cox regression analysis, we found that high serum YKL-40 level was an independent predictor of MACE after controlling for clinical and angiographic variables (hazard ratio: 1.65, 95% confidence interval: 1.14-2.39, P=.008). The results of our study indicate that serum YKL-40 may be used as a biomarker to predict the long-term outcome after PCI in patients with STEMI.