Objective To investigate the quantity and function of CD8+T cells in peripheral blood of pa-tients with repeated implantation failure(RIF).Methods Thirty-seven patients with RIF and 19 healthy controls were enrolled in this study.The peripheral blood and endometrium were collected during the mid-luteal phase.The percentage of peripheral CD8+T subsets and the levels of perforin and granzyme B of peripheral CD8+T cells were determined by flow cytometry assay.The percentage of endometrial CD8+T cells was detected by IHC,the produc-tion of perforin and granzyme B of endometrial CD8+T cells was detected by IF. Results Compared with the con-trol group,the percentage of peripheral CD8+T cells in patients with RIF was not significantly changed(37.22% vs. 37.15%,P>0.05).However,the porportion of endometrial CD8+T cells in the RIF group was higher than that in the control group(1.99% vs.3.77%,P<0.001).The levels of perforin and granzyme B in peripheral blood and en-dometrial CD8+T cells in patients with RIF were similar with those in the control group.Conclusions Compared to the control group,the percentage of endometrial CD8+T was markedly upregulated in patients with RIF.However, the production of perforin and granzyme B were similar between the control group and the RIF group.