OBJECTIVE: Rheumatoid arthritis (RA) is a common autoimmune disease that is primarily driven by effector T cells, particularly Th17 cells, which are mainly contained within CD4+CD161+ T cells. Thus, we aimed to explore whether the frequencies of circulating IL-17-producing CD4+CD161+ T cells and CD4+CD161+ T cells were correlated with RA disease activity. METHODS: The surface phenotype and cytokine production of blood were analyzed by flow cytometry in 52 RA patients and 17 healthy controls. The disease activity was evaluated by the 28-joint disease activity score. RESULTS: The frequencies of circulating IL-17-producing CD4+CD161+ T cells and CD4+CD161+ T cells were increased in RA patients, and they were elevated in patients with active disease status compared to patients with low disease status. Furthermore, their frequencies were positively correlated with disease activity parameters. Receiver operating characteristic curve analysis revealed that IL-17-producing CD4+CD161+ T cell levels were able to distinguish disease activity with 60.7 % sensitivity and 87.5 % specificity, while CD4+CD161+ T cell levels showed 92.9 % sensitivity and 66.7 % specificity. CONCLUSION: These results support the hypothesis that Th17 cells are involved in the pathogenesis of RA and suggest that circulating CD4+CD161+ T cells are a potential biomarker of RA disease activity.