A randomized study of mitomycin C (MMC)+5-FU [control group] vs MMC+5-FU+lentinan (LNT) [LNT group] was conducted in order to evaluate the effect of LNT against advanced gastric and colorectal cancers by the envelope method in 166 patients, comprising of 115 cases of gastric cancer and 51 cases of colorectal cancer. Significant increases were observed in the survival rates for the LNT group (p less than 0.05) for both patients with gastric and colorectal cancer. No significant difference was observed in response rates between the aforesaid two groups, though the response rate in the LNT group was slightly higher than the control group. These results suggest that LNT in combination with anticancer drugs prolong the survival time of patients with advanced gastric and colorectal cancer.