Objectives: To study variation in T lymphocyte subgoups and its clinical significance in non-small cell lung cancer (NSCLC). Methods: Levels of CD3+, CD4+, CD8+, CD4+/CD8+, NK and Treg cells in peripheral blood of NSCLC cases and healthy adults were determined by flow cytometry. Results: CD3+, CD4+ and CD4+/CD8+ ratio and NK cells in NSCLCs were decreased significantly in comparison with the control group (P < 0.01), and decreased with increase in the clinical stage of NSCLC, while CD8+ cells demonstrated no significant change (P > 0.05). Treg cells were significantly more frequent than in the control group (P < 0.01), and increased with the clinical stage of NSCLC. Conclusion: The cellular immune function of the NSCLC patients is lowered. It is important to detect change of T lymphocyte subgroups by flow cytometry for the diagnosis, treatment and prognostic assessment of NSCLC patients.