Proinflammatory cytokines are deemed to play a pivotal role in the pathogenesis of multiple sclerosis (MS). They provide signals for T-cell activation and inflammatory cell recruitment in the brain and might directly alter neuroglial and neuronal cell survival and function. We found that peripheral blood mononuclear cells (PBMCs) from MS patients spontaneously produce high levels of TNFα, TNFβ, IFNγ, and oncostatin M (oncM), a proinflammatory cytokine acting on cells of neural, vascular, hematopoietic, and lymphoid origin. Spontaneous production of these cytokines was significantly higher (p<0.01) in PBMC short-term culture supernatants from MS patients than in blood donors (HC). On average, lectin-induced production of these cytokines by PBMC was higher in MS patients than in HC, significantly so only for TNFα (p=0.013). Determination of TNFα, TNFβ, IFNγ, and oncM in corresponding sera showed that, on average, oncM levels were higher in MS patients than in HC, though the results were not statistically significant, whereas levels of TNFα, TNFβ, and IFNγ were below the assay threshold in most patients. The finding that MS PBMCs are primed in vivo to produce and release high levels of proinflammatory cytokines suggests the presence of a basal activation of the immune system which, in turn, may play a role in the complex circuitry of molecular and cellular interactions responsible for neurologic damage in MS.