Psoriasis is characterized by a dermal and epidermal infiltrate comprised predominantly of CD4⁺ and CD8⁺ T cells, respectively. These cells behave in an antigen-dependent manner, which suggests that psoriasis may be a T-cell-mediated autoimmune disease. Psoriasis shares certain immunological features with recognized autoimmune conditions such as type I diabetes mellitus and multiple sclerosis, in both of which a pathogenic role is postulated for natural killer (NK) cells and natural killer-like T (NK-T) cells. However, there are few studies assessing the role of NK and NK-T cells in psoriasis. We sought to determine whether NK and NK-T cells are present in psoriasis. Skin biopsies were taken from the active edge of a psoriasis plaque and from uninvolved skin at least 5 cm away from involved skin of ten patients with chronic plaque psoriasis. Skin from four normal subjects was used as controls. Using an immunoperoxidase technique, cryostat sections were stained using antibodies to T-cell markers CD2, CD3, CD4 and CD8; cutaneous leucocyte associated antigen; NK cell markers CD16, CD56, CD57, CD94 and CD158a; and the NK-T cell marker CD161. There were significantly more cells expressing T cell markers, NK cell markers CD16, CD57, CD94 and CD158a and NK-T cell marker CD161 in involved skin than in uninvolved or normal skin (P<0.01). There was no difference in the number of cells expressing CD56. Cells expressing NK markers were found most commonly in the papillary dermis immediately subjacent to the dermoepidermal junction. Cells expressing CD57 were found in significantly higher numbers in the epidermis and reticular dermis of involved skin. This study demonstrates that cells expressing NK markers and NK-T cell markers are present in plaques of psoriasis. The exact roles of NK and NK-T cells in psoriasis are unclear, although they may modulate autoimmune inflammation and act as a source of Th₁ cytokines important in the psoriatic process.