High doses of glucocorticoids (GCs) are widely employed to treat acute attacks in relapsing-remitting multiple sclerosis (MS) patients. Their beneficial effects are partially due to their capacity to regulate the cytokine network. In the present work, we have examined the effect of GCs on the production of the immunosuppressor cytokine IL-10. Blood samples from MS patients suffering an acute relapse were obtained immediately before initiating therapy and after receiving a daily dose of 1 g intravenous methylprednisolone (MP) for four days. Levels of IL-10 mRNA in PBMC were semiquantified by RT-PCR, whereas protein concentration in serum and in cell culture supernatant was measured by ELISA. Our results show that 7 out of the 9 patients studied displayed increased IL-10 mRNA expression as well as higher serum IL-10 concentration following steroid treatment. In contrast, mRNA expression of two inflammatory cytokines, TNFalpha and IFNgamma, decreased following steroid therapy. In vitro experiments employing normal PBMC showed that methylprednisolone (MP) upregulated IL-10 expression as determined by measuring mRNA levels, flow cytometry of intracytoplasmic protein concentration, and the amount of secreted protein. Peak responses of secreted IL-10 by PBMC cultured cells treated with MP were obtained at 48 h. The effect was steroid-specific as IL-10 expression reversed to baseline levels in the presence of the glucocorticoid receptor antagonist RU486. Contrary to the effect of MP on the spontaneous expression of IL-10, this drug downregulated LPS-induced IL-10 synthesis. In fact, the concentration of IL-10 in LPS-induced IL-10 secretion from normal PBMC decreased upon addition of MP to cell cultures. Thus, it seems that MP exerts an opposite effect on the spontaneous and LPS-induced IL-10 production. Our studies indicate that GCs may control inflammatory responses by upregulating production of the immunosuppressor cytokine IL-10.