A prospective, randomized multi-center study was conducted to assess the clinical effectiveness of Lentinan, an immunomodulatory agent, in the metastatic prostate cancer. Of seventy-five patients enrolled from July 1987 to June 1992, 69 were eligible. All patients received hormonal therapy and chemotherapy using Tegafur p.o. at a dose of 400-800 mg/day. While 33 patients received Lentinan i.m. for at least three months, the other 36 did not. The dose of Lentinan was 2 mg weekly for inpatients and 4 mg every other week for outpatients. The mean age of treated and control patients was 70 (range; 53-83) and 71 (range; 50-86), respectively. The 50% survival length of treated and control patients was 48 and 35 months, respectively. The five-year survival rate of treated patients was 43% according to the Kaplan-Meier method, while that of control patients was 29% (p < 0.05). We conclude that Lentinan is effective in metastatic prostate cancer when incorporated into hormonochemotherapy.