The tumorigenicity of hydrogen peroxide (HP) was examined by administering 0.4% HP solution in drinking water to various strains of mice. In C57BL mice, gastric lesions in the forestomach occurred in over 67% of the mice treated with HP for 120 days and duodenal lesions were noted in over 80% of the mice that received HP for 60 days. Gastric lesions were found only in the glandular stomach and duodenal lesions were restricted to the peri-pyloric and proximal part of the duodenum. The lesions in both organs were often multiple. The gastric lesions were composed of erosion and hyperplasia and those in the duodenum were exclusively hyperplasia. In mice given 0.4% and 0.1% HP for 420 to 740 days, 5 and 1% of them, respectively, had duodenal cancer by histological criteria though they did not show any distant metastasis. In the control group, no cancer was noted in the same observation period. The incidence and average numbers of lesions in both the stomach and duodenum after 150 to 210 days of HP intake decreased or fell to zero if HP administration was interrupted for 10 to 30 days. In DBA and BALB mice given 0.4% HP for 90 to 210 days, the incidences of gastric lesions reached 30 and 10% at maximum, respectively, and the incidences of duodenal lesions were 60 to 100% and 40 to 69%, respectively.