Summary We have studied the induction of papilloma formation in response to skin tumor promotion in transgenic mice overexpressing the human ornithine decarboxylase gene and in their nontransgenic littermates. The transgenic animals displayed a basal epidermal ornithine decarboxylase activity that was nearly 20 times higher than in their nontransgenic littermates. A single topical application of 12- 0-tetradecanoylphorbol-13-acetate induced a much more profound and longer-lasting increase in transgene-derived ornithine decarboxylase activity in comparison with the endogenous enzyme activity. Initiation of skin tumorigenesis with a single topical application of dimethylbenz[a]antracene followed by twice-weekly application of 12- 0-tetradecanoylphorbol-13-acetate resulted in the appearance of first papillomas both in nontransgenic and transgenic animals by week 7. However, after 11 weeks of 12- 0-tetradecanoylphorbol-13-acetate application, the number of papillomas per animal was almost 100 % higher in the transgenic animals than in their nontransgenic littermates. These results indicate that an overexpression of epidermal ornithine decarboxylase confers a growth advantage on skin tumors in vivo.