The environmental contaminant benzo[a]pyrene (B[a]P) has been regarded as one of the pathogens of oral premalignant and malignant lesions. To elucidate the pathogenesis of oral premalignancies, B[a]P-induced dysplasia of the murine tongue was investigated for G1-associated cell cycle regulation. B[a]P solution was applied orally up to six weeks to induce epithelial dysplasia of the tongue. BrdU incorporation and the expression of p21, cyclin D1, and CDK4 were examined by immunohistochemistry and Western blotting. Rb phosphorylation and E2F-Rb binding were examined by immunoprecipitation and Western blotting. B[a]P treatment resulted in dysplastic changes and active DNA synthesis in the tongue epithelia. Immunohistochemical analyses showed p21 up-regulation and cyclin D1/CDK4 overexpression in B[a]P-induced dysplasia. Rb hyperphosphorylation and E2F release were caused by B[a]P treatment. Thus, dysregulation of G1-phase regulation is likely to be an important event in the development of oral epithelial dysplasia in mice.