We examined the effectiveness of postoperative adjuvant chemotherapy with mitomycin C (MMC), 1-(2-tetrahydrofuryl)-5-fluorouracil (tegafur), plus PSK, an immunomodulator, for patients with advanced gastric cancer who underwent histological curative resection. The effect of chemotherapy on prognostic serosal (ps) invasion [ps(-) or ps(+)] and lymph node metastasis [n(-) or N(+)] was examined. One hundred eighteen patients were in the no-chemotherapy group and 137 were on the drugs. The median follow-up time for the 86 survivors at the time of analysis was 13.8 years. With regard to prognostic factors, there were no differences between the two groups. Generalized Wilcoxon test of the two survival patterns revealed a P value of .0351, and the survival rate for 15 years was 45.7% for patients in the no-chemotherapy group and 56.9% for those of the chemotherapy group. In particular, adjuvant chemotherapy was effective for patients with ps(-)n(+) (P less than .05) and ps(+)n(-) (P less than .05), but not for those with ps(-)n(-) and ps(+)n(+). Our findings show that the concomitant prescription of MMC, tegafur, and PSK improves the 15-year survival rate for patients with advanced gastric cancer, following curative resection. As the survival rate is low for the patients with ps(+)n(+), an even more aggressive postoperative chemotherapy is recommended.