Pyrimidine dimers participate as important factors in ultraviolet-induced lethality, mutagenicity and tumorgenicity. Substantial efforts have been made in recent years to understand the induction of pyrimidine photodimers and their repair in human skin cells exposed to low physiological fluences of UV-light. Dimers are known to be efficiently induced after UVC and UVB irradiation, but these photoproducts are also highly induced in DNA isolated from human skin irradiated with UVA. By using a sensitive immunohistochemistry dimer detection assay, we confirm that in vivo UVA radiation induces substantial amounts of these DNA changes in the epidermis; in addition, this technique detects them far into the reticular dermis. A considerable number of these photodimers were also seen in non-irradiated control skin up to two centimeters from the irradiation site. All the lesions persist for at least two days post-irradiation. These results sustain the hypothesis that pyrimidine dimer formation and excision could be a modality of epidermal communication.