The level of soluble interleukin-2 receptor (sIL-2R) was quantitated with enzyme-linked immunosorbent assay in serum and cerebrospinal fluid obtained from 24 patients with multiple sclerosis and 10 patients with other neurological disorders in whom immunological mechanisms are unlikely to participate. The sIL-2R level in the serum and cerebrospinal fluid of patients with multiple sclerosis in relapse was significantly higher compared with patients with multiple sclerosis in remission and with controls. The sIL-2R level, especially in the cerebrospinal fluid, showed higher sensitivity and specificity than other clinical parameters including the cerebrospinal fluid IgG ratio, peripheral lymphocyte CD4/CD8 ratio, cerebrospinal fluid myelin basic protein and oligoclonal bands. Our data suggest that measurement of the sIL-2R level may be useful in evaluating disease activity in patients with multiple sclerosis.