Irinotecan hydrochloride (CPT-11) is a potent anti-cancer drug with suppressive effects against gastric and colorectal cancers. However, no evaluation of CPT-11 for esophageal squamous cell carcinoma has been performed in vivo. In this study, we examined the tumor suppressive effects of CPT-11 on N-nitrosomethylbenzylamine (NMBA)-induced rat esophageal tumorigenesis. One hundred and fourteen rats were divided into six groups. Rats in groups 1-5 were treated with subcutaneous injection of NMBA at a dose of 0.5 mg/kg 3 times per week for 5 weeks. Rats in group 6 served as a control. Rats in groups 2 and 3 were treated with intragastric (i.g.) administration of CPT-11 at doses of 20 and 40 mg/kg, respectively, once a week simultaneously with NMBA-initiation up to the end of the experiment. Rats in groups 4 and 5 were treated with i.g. administration of CPT-11 at doses of 20 and 40 mg/kg, respectively, once a week after the NMBA-initiation period up to the end of the experiment. The incidence of papilloma and hyperplasia in the esophagus showed no difference between NMBA-treated groups. However, the multiplicity of hyperplasia was significantly reduced in all CPT-11 administration groups. Immunohistochemically, expression of proliferating cell nuclear antigen (PCNA) was decreased on carcinogen-exposed squamous epithelium and preneoplastic lesions, although no significant differences were detected in the expression of single-strand DNA (ssDNA) and p53. These data suggest that CPT-11 has suppressive effects for esophageal tumors in the early step of the multistep process of carcinogenesis through antiproliferative mechanism.