Serum but not CSF concentrations of the interferon-gamma-inducing cytokine interleukin (IL)-18 were significantly augmented in patients with MS as compared to both healthy controls and patients with other neurologic diseases. Patients with MS with secondary chronic progressive disease had significantly higher serum levels than those with relapsing remitting MS. In the latter group, IL-18 levels were higher in patients with acute exacerbation as compared to those with stable disease.