Skin irradiation with ultraviolet B (UVB) is a common and often durable treatment for psoriasis and other inflammatory skin disorders. We studied the effects of UVB on keratinocytes and leukocytes in psoriatic tissue and in culture. In nine patients treated repetitively, most of the cellular and molecular changes that typify the psoriatic epidermis reverted to normal. Keratinocyte hyperplasia, assessed by expression of the Ki-67 cell cycle antigen, decreased by 70%, and residual cell proliferation was appropriately confined to the basal layer. Epidermal thickening was reduced by 60%, and a granular layer reformed. Expression of keratin 16, as well as suprabasal integrin alpha 3 and insulin-like growth factor-1 receptors, was eliminated, whereas filagrin increased markedly. UVB also depleted > 90% of the CD3+, CD8+, and CD25+ T cells from the psoriatic epidermis, whereas dermal T cells were only minimally depressed. The latter finding parallels the known inability of these doses of UVB to penetrate the dermis. In tissue culture, UVB was antiproliferative and cytotoxic toward T cells and keratinocytes, but the T cells were 10-fold more sensitive. Furthermore, low doses of UVB induced apoptosis in lymphocytes but not keratinocytes, as detected by the TUNEL (TdT-mediated dUTP-biotin nick end labeling) technique. The selective effects of UVB on intraepidermal T cells in situ and in culture support the hypothesis that epidermal alterations in psoriasis can be normalized by a depletion of activated intraepidermal T cells.