To understand the role of cutaneous immune cells in host resistance to the induction and growth of skin cancer, we investigated the number and morphology of murine dendritic epidermal cells (dEC) during the evolution of ultraviolet (UVA) UV-induced skin cancers. Female C3H/HeN mice were treated topically with 8-methoxypsoralen followed by ultraviolet A (UVA) radiation 3 times/week or irradiated with UVB radiation 3 times/week. In both psoralen plus UVA- and UVB-treated mice, ATPase+ and Ia+ Langerhans cells almost completely disappeared from the treated skin during the early latency period of tumor development (4 weeks) but reappeared in the epidermis late in the latency period (between 15 and 22 weeks). The ATPase+ cells that reappeared in the epidermis had a rounder, less dendritic morphology than normal Langerhans cells. Thy-1+ dEC were totally depleted from the epidermis in both treatment groups at the end of first week of treatment and were nearly absent from the skin during the entire latency period. After tumors appeared (29 weeks), Thy-1+ dEC were still absent or detected only in small numbers in skin surrounding the tumors. ATPase+ and Ia+ cells present in skin around the tumors constituted 60 to 80% of the number in nonirradiated skin. Mice that received UVA radiation alone developed no tumors. ATPase+ and Ia+ Langerhans cells and Thy-1+ dEC were detected in UVA-treated epidermis after 22 weeks and 43 weeks, although the numbers were lower than those in unirradiated mice. Most psoralen plus UVA-induced tumors (81%) were squamous cell carcinomas, whereas only 24% of UVB-induced tumors were of this histological type. Our results demonstrate that UV-induced skin cancers developed in the presence of ATPase+ and Ia+ cells in the epidermis and in the absence of Thy-1+ dEC.