We studied the effects of endoscopic intratumoral injection of Lentinan in 7 patients with advanced gastric cancer. Ten to 14 days before surgery, Lentinan at a dose of 3 mg was endoscopically injected into the cancer tissues. The effects of Lentinan injection were evaluated by immunohistochemical staining for lymphocyte subsets in the resected specimens and by the natural killer (NK) activity of peripheral blood lymphocytes before and after injection. The distribution of lymphocyte subsets in cancer tissues was compared with those of 7 patients with advanced gastric cancer without Lentinan injection (control group). The ratios of CD 8+ cells and CD 25+ cells to CD 3+ cells in cancer tissues were statistically higher in the group given Lentinan injection than in the control group. The NK activity of peripheral blood lymphocytes significantly increased from 16.0 +/- 4.6% before injection to 21.1 +/- 5.1% after injection. However, there were no changes in lymphocyte subsets during this period. There were no side effects caused by the Lentinan injection. We conclude that endoscopic intratumoral injection of Lentinan may enhance local and systemic immunity in patients with gastric cancer.