It is increasingly clear that the CD40 and CD40 ligand (CD40L) receptor-ligand pair mediates a crucial activation signal in both cell-mediated and humoral immune responses. Here, we detected mRNA levels of CD40 and CD40L in non-stimulated peripheral blood mononuclear cells in 46 patients with multiple sclerosis (MS) and 46 healthy controls by a competitive RT - PCR procedure allowing quantification without previous culture or antigenic stimulation. The levels of CD40 and CD40L mRNA were markedly increased in MS patients (P<0.0001) compared with healthy controls. There was no difference between clinical MS subgroups or stage of disease. Our findings indicate that, although MS is an organ specific disorder, an increased signaling via the CD40 and CD40L pathway may be present at the systemic level. The nature of this upregulation, whether primary or secondary to the organ-specific autoimmune response, is yet to be determined. Since interference with CD40/CD40L is an effective way to interfere with autoimmune model diseases such as experimental autoimmune encephalomyelitis, it may be relevant to investigate further the role of these molecules in the pathogenesis of MS.