In vivo ultraviolet irradiation of human skin results in profound perturbation of the immune system. Relevance to ultraviolet-induced skin cancer [published erratum appears in Arch Dermatol 1991 Sep;127(9):1368] Ultraviolet exposure of human skin deletes the function of antigen-presenting Langerhans cells normally resident within the epidermis. Langerhans cells are capable of activating T-lymphocytes by presenting antigens (such as nickel or tumor antigens) to T-lymphocytes. Such activated T-lymphocytes may be involved in the development of contact dermatitis and the immune surveillance of immunogenic skin cancers. Deletion of the function of Langerhans cells does not result in abrogated epidermal antigen presentation since ultraviolet irradiation simultaneously induces the appearance of another epidermal antigen-presenting cell population that is distinct from the Langerhans cell population and seems to induce suppression of the immune response. Suppression of the immune response following ultraviolet irradiation in murine models is critical for growth of immunogenic ultraviolet-induced skin neoplasm. Thus, ultraviolet irradiation of human skin may facilitate the growth of human neoplasms, and the spreading of skin-associated infections due to induction of suppressor T cells. 