Background. The use of statins is essential for aggressive lipid-lowering treatment in acute coronary syndrome (ACS) patients with dyslipidemia. Recently, elevation of sitosterol, a lipid absorption marker, was reported to be associated with premature atherosclerosis. The purpose of the present study was to examine the impact of ezetimibe, a selective intestinal cholesterol transporter inhibitor, in ACS patients. Methods. A total of 197 ACS patients were randomized to pitavastatin + ezetimibe (n = 100) or pitavastatin (n = 97). Low-density lipoprotein cholesterol (LDL-C) and sitosterol levels were evaluated on admission and after 12 weeks. Results. After 12 weeks, the pitavastatin + ezetimibe group showed a significantly greater decrease of sitosterol (baseline versus after 12 weeks; 2.9 ± 2.5 versus 1.7 ± 1.0 ng/mL, P < 0.001) than the pitavastatin group (2.7 ± 1.5 versus 3.0 ± 1.4 ng/mL). The baseline sitosterol level was significantly higher in patients with achieved LDL-C levels ≥ 70 mg/dL than in patients with levels < 70 mg/dL (3.2 ± 2.5 versus 2.4 ± 1.3 ng/mL, P = 0.006). Conclusions. Ezetimibe plus statin therapy in ACS patients with dyslipidemia decreased LDL-C and sitosterol levels more than statin therapy solo. Sitosterol Elevation was a predictor of poor response to aggressive lipid-lowering treatment in ACS patients.