Narrow-band ultraviolet B (UVB) phototherapy is an effective treatment for psoriasis. Owing to its limited penetration, the direct effects of UVB are mostly restricted to cells residing in the epidermis and papillary dermis, and are associated with epidermal depletion of Langerhans' cells (LC) and T cells. It has been argued that the depletion of the skin-resident T-cell population may be due to a combination of UVB-induced apoptosis and decreased recruitment from the blood due to lower expression of the required adhesion and homing molecules. We have previously demonstrated that UVB treatment can alter the expression of adhesion molecules by blood lymphocytes, and as these can be influenced by cytokines, the aim of this study was to investigate whether UVB irradiation can also influence the cytokine production of circulating T cells. Four patients with active chronic plaque psoriasis were treated daily with narrow-band 312 nm UVB irradiation and blood samples obtained before treatment and weekly thereafter for 2 weeks. Peripheral blood mononuclear cells (PBMCs) were isolated and cultured with a streptococcal superantigen or a conventional streptococcal antigen preparation, and cell culture supernatants were assayed for various cytokines. When stimulated with the superantigen, PBMCs from UVB-treated psoriasis patients secreted greater amounts of the anti-inflammatory cytokine IL-10, and showed markedly decreased production of IL-1beta, IL-2, IL-5 and IL-6 compared to the pre-treatment values; the production of IFN-gamma, IL-8 and IL-12p70 were also decreased but did not reach statistical significance. Thus, the combination of UVB-induced apoptosis, increased secretion of anti-inflammatory cytokines and decreased trafficking to the skin may help to explain the beneficial effects of UVB treatment on psoriasis and why disease remission can sometimes be sustained for a prolonged period.