Abstract: OBJECTIVE To detect the expression levels of Th and Treg cells and cytokines Interferon-γ(IFN-γ), Interleukin-4(IL-4), Interleukin-17(IL-17) and transforming growth factor-β(TGF-β) in elderly patients with pneumonia caused by multidrug-resistant organisms (MDROs), and explore the role of Th and Treg cell-mediated immune responses in elderly patients with pneumonia caused by MDROs. METHODS A total of 30 elderly patients with pneumonia caused by MDROs who were treated in the First Affiliated Hospital of Kunming Medical University during Aug. 2016 to Apr. 2018 were enrolled in the study. According to whether respiratory or circulatory support was needed, the patients were divided into the elderly severe pneumonia group and the elderly general pneumonia group, and 11 healthy elderly people in the same period were included as the control group. Peripheral blood mononuclear cells from the 30 elderly patients with pneumonia caused by MDRO and 11 healthy elderly peoples were isolated. The proportions of Th1, Th2, Th17 and Treg cells in CD4+ T cells were determined by flow cytometry. The expression levels of related cytokines IFN-γ, IL-4, IL-17 and TGF-β in serum were detected by enzyme-linked immune sorbent assay (ELISA). RESULTS The proportions of Th1 and Treg cells in CD₄⁺ T cells in severe pneumonia patients were (1.743±0.606)% and (0.978±0.271)%, respectively, significantly lower than those of the healthy control group (P<0.05), the expression levels of IFN-γ and TGF-β in serum were significantly lower than those of the healthy control group(P<0.05), and the expression of TGF-β in the severe pneumonia group was significantly lower than that in the general pneumonia group (P<0.05). The proportion of Th17 cells in CD₄⁺ T cells and the expression of IL-17A in the severe pneumonia group increased significantly as compared with the healthy controls (P<0.05). CONCLUSION Th1 and Treg-mediate cellular immunity was inhibited in elderly patients with pneumonia caused by MDRO, and Th17/Treg was imbalanced and related to the severity of the disease. This study lays foundation for the development of new strategies for anti-infective immunotherapy.