Purpose: To investigate the effect of systemic chemotherapy combined with vitamin E on the natural killer (NK) cell activity in patients with solid tumors. Patients and methods: Forty patients with various metastatic solid tumors were divided into 2 groups, each of which (n=20) received systemic chemotherapy for 3 months. In one group, vitamin E 200 mg/day given orally was added. Prior to therapy and following therapy NK cell activity was measured using flow cytometry and defined as percentage values. The results were analysed using Student's t-test and paired t-test. Results: In the systemic-chemotherapy alone group, NK cell activity significantly decreased following therapy (12.5 ± 6.1% versus 9.5 ± 5.3%, p < 0.001), whereas in the vitamin E-supplemented group, comparison of the values before and after therapy (9.1 ± 3.9% versus 12.5 ± 4.4%) and of those obtained from the chemotherapy-alone group following therapy, showed a significant increase (p < 0.001 for both comparisons). Conclusion: Administering vitamin E to cancer patients should be considered in order to lessen the adverse effects of systemic chemotherapy on the immune function.