Psoriasis is a type-1 T cell-mediated, chronic inflammatory disease. Since interleukin (IL)-12p70 promotes the development of type-1 T cells, we investigated whether psoriasis is associated with an increased production of this cyctokine by blood cells. Results revealed that the production of IL-12p70 by cells of psoriasis patients stimulated by 1 and 10 ng per mL, but not 100 ng per mL of lipopolysaccharide (LPS) was higher (p=0.03) than that by cells of healthy volunteers. The production of IL-12p40 by patients cells upon stimulation with 0.1 ng per mL LPS, but not higher concentrations, was higher (p=0.02) than that by cells of healthy volunteers. No association between IL-12p70 production by blood cells and the severity of psoriasis was observed, nor was there a difference in the LPS-stimulated production of this cytokine between cells of the early and late onset type of patients. The frequencies of the various genotypes for the promoter region of the gene encoding IL-12p40 (IL12B) did not differ between psoriasis patients and controls. No association was observed between the various IL12B promoter genotypes and the LPS-stimulated production of IL-12p70 or IL-12p40 by blood cells. Together, psoriasis is not associated with a promoter polymorphism in the IL12B gene nor with the production of IL-12p70 by LPS-stimulated blood cells.