Abstract: OBJECTIVE To explore the relationship between expression levels of costimulatory molecules OX40 and programmed death-1（PD-1） in peripheral blood and infection in elderly patients undergoing total hip arthroplasty（THA）. METHODS A total of 600 elderly THA patients admitted to Shangqiu First People's Hospital between Feb 2018 and Dec 2020 were enrolled. According to presence or absence of postoperative infection, they were divided into the infection group（41 cases） and non-infection group（559 cases）. The costimulatory molecules OX40 and PD-1 on the surface of peripheral blood mononuclear cells（PBMC） were detected on the 2 rd day after surgery. The influencing factors of postoperative infection were summarized. The predictive value of OX40 and PD-1 for postoperative nosocomial infection was assessed by receiver operating characteristics（ROC） curves. RESULTS Of the 600 elderly patients undergoing total hip arthroplasty, 41 cases（6.83%） had infection and 69 strains of pathogens were isolated, including 20 strains of gram-positive bacteria, 46 strains of gram-negative bacteria and 2 strains of fungi. Univariate and multivariate analysis showed that the time of drainage tube placement and diabetes mellitus had a certain influence on the infection after hip replacement in the elderly（P＜0.05）. The expression level of costimulatory molecule PD-1 on the surface of peripheral blood CD₄⁺ and CD₈⁺ T lymphocytes in the infection group was significantly higher than that in the non-infection group, while expression level of OX40 was significantly lower than that in the non-infection group（P＜0.05）. The area under the ROC curve（AUC） of the combined detection of OX40 and PD-1 on the surface of peripheral blood CD₄⁺ and CD₈⁺ T lymphocytes for predicting postoperative infection in elderly THA patients was 0.838, which was significantly higher than the single detection of them, with the AUCs of 0.748, 0.677, 0.720 and 0.664（P＜0.05）. CONCLUSION The decreased expression level of costimulatory molecule OX40 and increased level of PD-1 on the surface of peripheral blood T lymphocytes are closely related to the occurrence of postoperative infection in elderly THA patients, which may have high predictive value for postoperative infection.