Abstract: Objective:To elucidate the effect of chemotherapy on cellular immune function of patients with nasal Natu -ral Killer (NK)/T cell lymphoma through detection of the activity of T-lymphocyte subgroups and NK cells in peripheral blood of these patients before and after chemotherapy.Methods:A total of76cases in this controlled study were divided into 2 groups, i.e., the experimental group (EG) (41patients with nasal NK/T cell lymphoma) and the normal control group (CG) (35healthy participants without malignant tumors). Activity of the T-lymphocyte sub-groups and cell count of the NK cells in peripheral blood of the patients in the EG were each tested before chemotherapy and 10days after 2 cycles of chemical therapy. A statistical analysis of the related experimental results was conducted in the 2 groups. Results: In a comparison of the 2 groups, the proportion of the T-lymphocyte sub-groups was indiscriminate in the nasal NK/T cell lymphoma patients before chemotherapy, the proportion of T-lymphocyte subsets (CD 3 + , CD 4 + and the ratio of CD 4 +/CD8 + ) and values of Natural killer cells in patients with Nasal natural killer/T cell Lymphoma were significantly lower than that of the CG. The pro-portion of CD 3 + and CD4 +, and the specific values of CD4 +/CD8 +, as well as the proportion of NK cells greatly decreased, and the proportion of CD8 + and CD4 + CD25+ T-regulatory cells greatly increased compared with the control group (P < 0.05). After 2 cycles of chemotherapy, the values of CD 3 +, CD 4 +, Natural killer cells, and the ratio of CD 4 +/CD8 + were higher, while the values of CD 8 + and CD4 + CD25+ T-regulatory cells were lower compared to those in the group of pa-tients with improved condition (P<0.05). After the chemotherapy, apart from the proportion of CD4 + and CD8 +, there was no statistical significance in the differences between the other indicators in the group with improved patient condition and those in the control group ( P>0.05). However, in the patients with ineffective chemotherapy, the post-chemotherapeutic CD4 + and CD8 + proportions, CD4 +/CD8 + ratio and the number of NK cells further decreased compared to those before chemotherapy (P<0.05). Conclusion:Cellular immunity was suppressed and the ratio of T-lymphocyte subsets was vastly disordered in the patients with nasal NK/T cell Lymphoma. NK cells were remarkably decreased. By killing the tumor cells and inducing the apoptosis of tumor cells, effectual chemotherapy can lessen the tumor load, reduce the CD 4 + CD25+ T-reg cells, remove some immunosuppressive agents and improve cellular immunity, while the function of cellular immunity continues to worsen in the patients with failure of the chemotherapy. Detection of the T lymphocyte subsets and the change of NK cell counts may reflect the status of cellular immunity in patients, which is of great importance and significance in guiding clinical therapy and determining the prognosis.