Multiple sclerosis (MS) is a multifactorial and polygenic disorder of the central nervous system, its development being under strong influence of T-helpers type I which produce anti-inflammation cytokines. Interleukin 12 (IL12) plays a key role in such polarization of the immune response. Genotyping for polymorphism of the IL12B gene in the 3'-untranslated region, coding for the p40(IL12B) subunit, has been carried out in 62 patients with MS and 129 healthy controls. The C/C genotype frequency was twice higher in patients as compared to the controls (33.9% and 17.4%, respectively). The allele C in patients was associated with shorter duration of the first remission (p = 0.028) which was 1.79 +/- 0.28 in those with the C allele and 3.27 +/- 0.68 in other patients. Mean rate of relapses per year was also higher (p = 0.079) in patients with the C allele (0.96 +/- 0.11) comparing with the A allele (0.72 +/- 0.11). During the treatment with copaxone, a trend towards increasing of the time before the first relapse was observed in patients with the C allele. An analysis of immunologic indices revealed that they changed in opposite directions depending on the gene variant. The C-allele is suggested to have relation both to liability to MS and to its pathogenesis.