Abstract: OBJECTIVE To explore the risk factors for multidrug-resistant Pseudomonas aeruginosa pulmonary infection and observe the expressions of helper T cell 17 (Th17) and regulatory T cell (Treg) in the elderly patients. METHODS A total of 166 elderly patients with P. aeruginosa pulmonary infection who were treated in the Second Affiliated Hospital of Shandong First Medical University from Jan 2019 to Nov 2021 were recruited as the research subjects and were divided into the MDRO group with 48 cases and the non-MDRO group with 118 cases according to the fact whether the strains were multidrug-resistant organism (MDRO). Meanwhile, 100 healthy people who received physical examination were chosen as the control group. The percentages of Th17 cell and Treg cell in CD₄⁺ T cells, expression levels of interleukin (IL)-10 and IL-17A were detected. Multivariate logistic regression analysis was performed for the risk factors for the multidrug-resistant P. aeruginosa pulmonary infection in the elderly patients. The predictive value of the model was analyzed by means of receiver operating characteristic (ROC) curve. RESULTS The result of multivariate analysis showed that the endotracheal intubation, exposure to carbapenemases, use of no less than 3 types of antibiotics, high expressions of Th17 cell and IL-17A, and low expressions of Treg cell and IL-10 were the risk factors for the multidrug-resistant P. aeruginosa pulmonary infection in the elderly patients (P＜0.05). The regression equation model was established based on the risk factors, and the area under curve (AUC) was 0.896(95%CI:0.815-0.976)in prediction of multidrug-resistant P. aeruginosa pulmonary infection, with the sensitivity 92.8%, the specificity 86.6%. CONCLUSION The multi-drug resistance of the elderly patients with P. aeruginosa pulmonary infection is associated with the invasive operation, use of multiple types of antibiotics and Th17/Treg immune imbalance. The combination of the multiple risk factors may have high value in prediction of multi-drug resistance.