The incidence of skin cancer is increasing in the United States, and effective primary prevention and chemoprevention strategies are needed to reduce its incidence, morbidity and mortality. Squamous cell carcinoma, in contrast to most internal cancers, is characterized by a high rate of p53 mutations and p53 overexpression in premalignant actinic keratosis (AK). Similarly, p53 mutations are frequently found in normal-appearing, chronically sun-exposed skin. We have been using p53 mutations and p53 overexpression as a potential biomarker of response to chemoprevention agents. Difluoromethylornitine (DFMO), an irreversible inhibitor of ornithine decarboxylase, inhibits the synthesis of polyamines, which have an important role in the process of skin carcinogenesis. In a randomized, placebo-controlled study of topical DFMO for six months in 48 subjects with AK, topical DFMO was effective in significantly reducing AK number (23.5%; P=0.001), spermidine levels (26%; P=0.04) and p53 protein expression (22%; P=0.04), but not cell proliferation. There were no significant differences in the frequency of p53 mutations (exons 5–8) (23.8% at baseline, 28.6% after placebo and 26.2% after DFMO). DFMO seemed to be more effective in reducing AK, spermidine and potentially p53 mutations on right arms (33%=one p53 mutation) as compared with left arms (67%=one p53 mutation), suggesting that left arms are more resistant to the effects of topical DFMO. The addition of biomarkers, developed on the basis of the molecular biology of the disease, to chemoprevention studies is essential.