Adjuvant immunochemotherapy using the antitumour polysaccharide sizofiran (SPG), an extract from the culture broth of Schizophyllum commune Fries, was prescribed randomly for 386 Japanese patients with resectable gastric cancer. Although the overall survival probability for 5 years did not differ between the SPG and control groups, in 264 patients with curatively resected cancer, the probability to 5 year survival and to recurrence in the sizofiran-administered patients was better than in the controls. In the multivariate analysis, four of six prognostic factors correlated with the prognosis of the 264 patients who underwent curative surgery, that is, nodal involvement (chi 2 = 21.426, P = less than 0.0001), age distribution (chi 2 = 9.262, P = 0.010), sizofiran administration (chi 2 = 6.507, P = 0.011), and primary tumour size (chi 2 = 9.345, P = 0.025). Thus, patients with a curatively resected gastric cancer had a better prognosis when sizofiran was prescribed in combination with antitumour drugs.