Objective:To analyze the correlation between peripheral blood lymphocyte subsets and unex-plained repeated implantation failure(URIF),and to investigate its predictive value for the diagnosis of URIF.Methods:A total of 156 patients with URIF who underwent treatment in the Department of Reproductive Medi-cine,The Second Affiliated Hospital of Kunming Medical University from October 1,2019 to December 1,2021 was selected as the URIF group.Meanwhile,age-matched 41 women with a history of healthy delivery in the past one year were selected as the control group.The percentages of peripheral blood lymphocyte subsets in the two groups were measured by flow cytometry,and the results were statistically analyzed.Receiver operating charac-teristic(ROC)curve was used to evaluate the predictive effect of lymphocyte subsets on URIF.Results:Com-pared with the control group,the percentage of CD3 + CD4 +[(34.03±7.26)%vs.(36.79±6.35)%,P = 0.017]、CD3 +HLA-DR +[(2.60±2.28)%vs.(3.60±2.39)%,P =0.017]、CD3 + CD16 + CD56 +[(1.24±1.04)%vs.(2.62±2.57)%,P<0.000]and CD4 +/CD8 +(1.37±0.48 vs.1.57±0.51,P =0.023)were sig-nificantly increased in URIF group,and were related to the increase of previous failure times to a certain extent.ROC analysis showed that CD3 + CD4 +>35.35%(AUC 0.624),CD3 + HLA-DR +>2.35%(AUC 0.669),CD3 +CD16 +CD56 +>1.86%(AUC 0.660)and CD4 +/CD8 +>1.26(AUC 0.628)could be used as an inde-pendent predictor for the diagnosis of URIF.Among the pair-wise combined indexes,CD3 + HLA-DR + combined with CD3 + CD16 + CD56 + had the highest prediction accuracy(AUC 0.739,Sensitivity 73.3%,Specificity 68.3%).The combination of the four indicators had the highest accuracy(AUC0.767,Sensitivity 68.6%,Specifici-ty 73.2%).Conclusions:There is immune dysfunction in patients with URIF,and the imbalance of peripheral blood lymphocyte subsets may be an important factor leading to embryo implantation failure.CD3 +CD4 +、CD4 +/CD8 +、CD3 +HLA-DR +and CD3 +CD16 +CD56 +could be used as independent indicators for the diagnosis of abnormal peripheral blood lymphocyte subsets in patients with URIF.The combination of them improves the accuracy of prediction,and it has a positive reference significance for dynamic monitoring and early intervention of URIF pa-tients in the process of assisted reproduction technology.