The aim of the study was to investigate the expression of miR-155 in plasma and peripheral blood mononuclear cells (PBMCs), the effects of miR-155 on the apoptosis rate of PBMCs and the extent of coronary stenosis in coronary artery disease patients. Seventy chest pain patients were divided into three groups by symptoms, signs, auxiliary examination and coronary arteriography: 21 cases in the acute myocardial infarction group (AMI), 23 cases in the Angor pectoris group (AP) and 26 cases in the control group (CT). The peripheral blood mononuclear cells of the patients in the three groups were separated for cell culture. Annexin V/propidium iodide (PI) was performed to analyze cell apoptosis. RT-qPCR was used to evaluate the expression level of miR-155 in plasma and PBMCs. There were significant differences on the expression of miR-155 in plasma and PBMCs, the apoptosis rate of PBMCs, Gensini score and the extent of coronary stenosis among the three groups (P<0.05). The expression of miR-155 in plasma and PBMCs in AMI and AP group were lower than CT group while the AMI group was lower than the AP group (P<0.05). The apoptotic rate of PBMCs, Gensini score and the extent of coronary stenosis of the AMI and AP groups were higher than CT group while the AMI group was higher than the AP group (P<0.05). The expression of miR-155 in plasma was positively correlated with PBMCs, but negatively correlated with the apoptosis rate of PBMCs, Gensini score and the extent of coronary stenosis. The apoptosis rate of PBMCs in patients with coronary heart disease was positively correlated with the degree of coronary artery stenosis and Gensini score. In conclusion, in the patients with coronary heart disease the apoptosis rate of PBMCs was increased and the expression of miR-155 in plasma and PBMCs cells was decreased, which were correlated with the severity of coronary heart disease.