In Japan the standard adjuvant treatment after resection of gastric cancer is intravenous mitomycin plus oral fluorouracil. We have assessed the efficacy of protein-bound polysaccharide (PSK) in addition to standard chemotherapy in patients who had undergone curative gastrectomy at 46 institutions in central Japan. 262 patients were randomly assigned standard treatment alone or with PSK. The minimum follow-up time was 5 years (range 5-7 years). PSK improved both the 5-year disease-free rate (70.7 vs 59.4% in standard treatment group, p = 0.047) and 5-year survival (73.0 vs 60.0%, p = 0.044). The two regimens had only slight toxic effects, consisting of nausea, leucopenia, and liver function impairment, and there were no significant differences between the groups. The treatments were clinically well tolerated and compliance was good. Addition of PSK to adjuvant chemotherapy with mitomycin and fluorouracil is beneficial as treatment after curative gastrectomy.