Objectives To test the antitumor potential of lymphocytes transferred via adoptive cell therapy (ACT) in a mouse model of human gastric cancer (GC), and to evaluate the clinical efficacy and safety of combining lymphocytes as adjuvant therapy with first-line chemotherapy in patients with GC. Methods We constructed a human GC xenograft model in sublethally irradiated 6-8-week-old male NCG mice. MKN-45 cells (1 x 10(6) cells/mouse) were subcutaneously injected into mice's flanks. After tumors had become palpable, we randomized the mice into control, ACT(IL-2), and ACT(IL-15) groups. Human lymphocytes were then injected into mouse tail veins. In addition, 63 human patients with histologically or cytologically confirmed stage III-IV GC randomly received S-1 + oxaliplatin + ACT(IL-15) (combination therapy group) or S-1 + oxaliplatin (chemotherapy group). Results In the mouse study, treatment with ACT(IL-15) cells inhibited tumor growth on adoptive transfer, and mice that received ACT(IL-15) cells had significantly longer survival rates (p < 0.05, ACT(IL-15) vs. ACT(IL-2)). In the human study, the median survival rate of patients in the combination therapy group was 472 days (95% confidence interval [CI], 276-668 days), whereas that of patients in the chemotherapy group was 266 days (95% CI, 200-332 days; p < 0.05). Eleven percent (7/63) of patients had adverse reactions, but these reactions did not interfere with treatment. Conclusion Adoptive transfer of ACT(IL-15) cells in a mouse model of GC and in patients with advanced GC treated with S1 + oxaliplatin improved survival rates in both, with an acceptable safety profile.