Immunopotentiators are expected to intensify the effects of chemotherapy, but anticancer agents are in greater or lesser degree carcinogenic in humans. The carcinogenic risk after adjuvant chemo-immunotherapy was examined in 786 gastric cancer patients who underwent curative gastrectomy between 1963 and 1981. They consisted of 420 patients without any chemo- or immunotherapy (cont group), 302 with chemotherapy (chem group) with mitomycin C and/or fluorinated pyrimidines, and 64 with chemo-immunotherapy (chem-immu group) with both chemotherapy and immuno-potentiator (s), levamisole or PSK and/or OK-432. The incidence of cancers other than gastric cancer 5 or more years after gastrectomy was 9 out of 64 in the chem-immu group (14.1%), which was higher than the 25 out of 302 in the chem group (8.3%) and significantly higher than the 27 out of 420 in the cont group (6.4%) (p < 0.05). The 5/10-year survival rate after gastrectomy was 76.0%/72.5% in the chem-immu group, which was significantly higher than the 66.1% /59.4% in the chem group and the 64.7%/59.3% in the cont group (p < 0.05). Among 215 patients under 50 years of age, the survival rate was 72.7%/65.6% in the chem group, which was significantly lower than the 75.0%/71.7% in the cont group (p < 0.01) and lower than the 81.9%/81.9% in the chem-immu group. However, the incidence of a second malignancy showed a tendency to increase in the order of the cont, chem and chem-immu groups.