Background and objective Th17 cells are important T helper cells, which are characterized by their production of IL-17. hT17 cells play an important role in host defense against microbial infections, autoimmune diseases and cancer. hTe aim of this study is to investigate the percentage of hT17 in peripheral blood lymphocyte and the level of IL-17 in serum and cerebrospinal lfuid (CSF) in patients with brain metastases from lung cancer. Methods Twenty-two patients with brain metastases from lung cancer and 20 health controls were analyzed. hTe percentage of hT17 cell was detected with lfow cytometry using CD3+CD4+IL-23R+marker, the level of IL-17 was measured with ELISA method. Results hTe percentage of hT17 cells in patients with brain metastases from lung cancer was 4.65%±0.72%, which was remarkably higher than that in con-trols (2.71%±0.54%, P=0.04). hTere was no signiifcant difference between non-small cell lung cancer (NSCLC) patients and small cell lung cancer (SCLC) patients. Serum IL-17 was remarkably increased in patients with brain metastases from lung can-cer (117.4±16.43 pg/mL vs 72.55±8.19 pg/mL, P=0.02). No signiifcant difference of the serum IL-17 was observed between NSCLC and SCLC patients. hTe level of IL-17 in CSF from patients with brain metastases from lung cancer was signiifcant higher than that from lung cancer patients without brain metastases (73.21±7.52 pg/mL vs 50.25±8.04 pg/mL, P=0.04). Con-clusion hT17 cells and IL-17 increase in patients with brain metastases from lung cancer. It may involve in the pathogenesis of brain metastases from lung cancer.