Background There are numerous articles on the endothelial progenitor cells (EPCs) in different disease conditions.However, the functional properties of EPCs in acute coronary syndrome (ACS) are still uncertain. Here we aimed to study the number and functions of EPCs in ACS patients.Methods Patients were enrolled with admitted ACS (n=25) and another 25 gender-, age-, atherosclerotic risk factors-matched stable coronary artery disease (CAD) controls. EPCs were defined as CD34+/CD133+/VEGFR-2+ and quantified by flow cytometry. Moreover, functional properties of EPCs including colony-forming unit (CFU), proliferation,migration as well as apoptosis were evaluated and compared between the two groups. Plasma matrix metalloproteinase-9 (MMP-9) was detected in all patients as well.Results The two groups had similar medication and clinical characteristics on admission. The EPCs in ACS patients were more than 2.6 times that in stable CAD subjects (15.6±2.7 vs. 6.0±0.8/100 000 events, P ＜0.01). CFU was not statistically different between the two groups (10.8±2.9 vs. 8.2±1.8, number/well, P ＞0.05). Furthermore, EPCs isolated from ACS patients were significantly impaired in their proliferation (0.498±0.035 vs. 0.895±0.067, OD value, P ＜0.01) and migration capacity (20.5±3.4 vs. 30.7±4.3, number/well, P ＜0.01) compared with controls. Moreover, the apoptosis cell in cultured EPCs was drastically increased in ACS group ((18.3 ±2.1 )% vs. (7.8±0.4)%, P ＜0.01 ).Conclusions Patients with ACS exhibited apparently increased circulating EPCs as well as cultured apoptosis percentage together with a remarkable impairment of proliferation and migration activities compared with stable CAD subjects.