BACKGROUND AND OBJECTIVE Th17 cells are important T helper cells, which are characterized by their production of IL-17. Th17 cells play an important role in host defense against microbial infections, autoimmune diseases and cancer. The aim of this study is to investigate the percentage of Th17 in peripheral blood lymphocyte and the level of IL-17 in serum and cerebrospinal fluid (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. The percentage of Th17 cell was detected with flow cytometry using CD3+CD4+IL-23R+ marker, the level of IL-17 was measured with ELISA method. RESULTS The percentage of Th17 cells in patients with brain metastases from lung cancer was 4.65%±0.72%, which was remarkably higher than that in controls (2.71%±0.54%, P=0.04). There was no significant 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 cancer (117.4±16.43 pg/mL vs 72.55±8.19 pg/mL, P=0.02). No significant difference of the serum IL-17 was observed between NSCLC and SCLC patients. The level of IL-17 in CSF from patients with brain metastases from lung cancer was significant 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). CONCLUSIONS Th17 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.