Psoriasis is a common chronic skin disorder affecting 2% of the general population. Present evidence strongly suggests that it is an immunologically mediated disease; the evidence includes the results of disease association studies linking psoriasis to certain MHC antigens and immunohistochemical studies revealing early influx into lesions of activated T lymphocytes. Accumulation of these cells in skin is mediated by upregulated expression of leukocyte adhesion molecules on vascular endothelium and epidermal keratinocytes and by production of proinflammatory and chemotactic cytokines. Activation of cell-mediated immune mechanisms in lesional skin is highlighted by the increased antigen-presenting capacity of Langerhans cells isolated from psoriatic skin compared to normal skin. The nature of the antigens precipitating psoriasis, however, remains unknown although a role for streptococcal superantigens has been postulated. These studies have led to the belief that immunotherapy may hold great promise for the treatment of psoriasis. Indeed both cyclosporin A and FK506 are effective therapies and evidence suggests that anti-CD4 antibodies may be of great value.