IL-18 is a cytokine which plays an important role in Th-1 response through its ability to induce IFN-gamma production in T cells and NK cells. The purpose of the study was to measure IL-18 levels in serum and CSF of 21 patients with the relapsing-remitting form of MS, 9 with active gadolinium enhancing lesions in MRI and 12 without enhancing lesions, and to compare results with control group consisting of 11 patients with diagnosis of neurasthenia and tension headache. IL-18 concentration in the CSF and sera was measured by ELISA. We found a highly significant increase of both IL-18 CSF and serum levels in MS patients in comparison with the control group. In patients with active MRI lesions the levels of IL-18 in CSF and serum were significantly higher in comparison with the levels found in patients without enhancing lesions. The results suggest involvement of IL-18 in immunopathogenesis of MS especially in the active stages of the disease.