Blood mononuclear cells (MNC) from patients with psoriasis were more adherent to monolayers of endothelial cells prepared from human umbilical cord vein than otherwise similar cells from control subjects. This increase in adherence occurred in the presence (mean 37% increase; p less than 0.01) and absence (mean 47% increase; p less than 0.05) of 10% autologous serum and was not related to the disease severity of the patients. The augmented adhesiveness of the patients' cells was also apparent when using monolayers of endothelial cells isolated from human skin. The levels of immune complexes, complement, alpha 2-macroglobulin, acute phase proteins (alpha 1-acid glycoprotein, C-reactive protein and alpha 1-antitrypsin), and tumor necrosis factor alpha (TNF alpha), interleukin-1 alpha (IL-1 alpha), and interleukin-1 beta (IL-1 beta) in the patients' sera were within normal limits. When MNC were added to endothelial monolayers that had been incubated with either TNF alpha or the highest concentration of rIL-1 beta used in the study, both the patients' and control's cells exhibited a similar increase in attachment (p less than 0.01). Pretreatment of endothelium with interferon-gamma did not enhance the attachment of MNC from either group of subjects. The augmented adherence of the patient's MNC appears to be due to an abnormal adhesiveness of the lymphocytes rather than the monocytes and is not related to an enhanced expression of the cell-surface adhesion molecules CD11a/CD18. It is likely that the circulating MNC of psoriatic patients may be predisposed for extravasation into skin.