Microbiological aspects are considered to be of pathophysiological importance in psoriasis, but there has so far been no information regarding cytomegalovirus (CMV) infection. This is of interest due to the high prevalence of latent infection in the general population, the frequent reactivation in inflammatory diseases, and the immunomodulating capacity of CMV. To detect active infection we analysed CMV antigen expression of peripheral blood mononuclear cells (PBMC) from psoriatic patients (n = 30) in comparison with healthy volunteers (n = 65). Using three monoclonal antibodies and immunocytological staining (alkaline phosphatase-antialkaline phosphatase technique), we frequently found CMV antigenaemia in psoriasis (43%) compared with healthy laboratory staff (12%, P < 0. 01) and blood donors (6%, P < 0.001). Clearance of CMV antigenaemia was observed with antipsoriatic treatment. CMV antigenaemia was symptomless, and was associated with seropositivity for anti-CMV IgG but not IgM antibodies, indicating subclinical activation of latent infection. Serological investigations in 85 psoriatic patients gave no evidence for a higher prevalence of latent CMV infection. In psoriatic lesions, CMV DNA was only rarely detected by polymerase chain reaction. As it has been shown that tumour necrosis factor (TNF)-alpha can induce CMV reactivation, we determined TNF-alpha plasma concentrations and mRNA expression in PBMC from psoriatic patients. Elevated TNF-alpha levels were found and correlated with the frequency of CMV antigen-expressing PBMC, suggesting a critical role of TNF-alpha in CMV activation. We speculate that active, subclinical CMV infection may be of pathophysiological importance in psoriasis.