Treatment with thymidine dinucleotide (pTT) has well documented DNA-protective effects and reduces development of squamous cell carcinoma in UV-irradiated mice. The preventive effect of pTT on basal cell carcinoma (BCC) was evaluated in UV-irradiated Ptch-1(+/-) mice, a model of the human disease Gorlin syndrome. Topical pTT treatment significantly reduced the number and size (P < 0.001) of BCCs in murine skin after 7 months of chronic irradiation. Skin biopsies collected 24 hours after the final UV exposure showed that pTT reduced the number of nuclei positive for cyclobutane pyrimidine dimers by 40% (P < 0.0002) and for 8-hydroxy-2'-deoxyguanosine by 61% (P < 0.01 compared with vehicle control). Immunostaining with an antibody specific for mutated p53 revealed 63% fewer positive patches in BCCs of pTT-treated mice compared with controls (P < 0.01), and the number of Ki-67-positive cells was decreased by 56% (P < 0.01) in pTT-treated tumor-free epidermis and by 76% (P < 0.001) in BCC tumor nests (P < 0.001). Terminal dUTP nick-end labeling staining revealed a 213% increase (P < 0.04) in the number of apoptotic cells in BCCs of pTT-treated mice. Cox-2 immunostaining was decreased by 80% in tumor-free epidermis of pTT-treated mice compared with controls (P < 0.01). We conclude that topical pTT treatment during a prolonged period of intermittent UV exposure decreases the number and size of UV-induced BCCs through several anti-cancer mechanisms.