Objective To explore the influence of proton pump inhibitors (PPI)omeprazole on the platelet aggregation rate of clopidogrel in patients with acute coronary syndrome (ACS).Methods A total of 159 patients with ACS were selected,based on the standardized treatment,the patients were divided into the two groups according to the illness.65 cases in the treatment group were given omeprazole (20mg/d).94 cases in the control group were not given omeprazole.Both two groups were given aspirin(100mg/d) and clopidogrel (75mg/d).The platelet reactivity index(PRI) and clopidogrel active metabolites (clopi-H4)were detected five days later.Results After treatment for 5 days,the number of PRI ＞ 50％ in the treatment group and control group was 12 cases(18.5％) and 7 cases (7.4％) (x2 =4.431,P ＜ 0.05).The clopi-H4 levels in the treatment group and control group were 332.8 mmol/L (SD 371.8) and 600.3 mmol/L(SD 885.2) (t =-2.298,P ＜ 0.05).Conclusion In patients with ACS,combined omeprazole and clopidogrel can reduce the concentration of clopidogrel active metabolites clopi-H4,inhibition of clopi dogrel platelet aggregation.