Anticardiolipin antibodies of the IgG, IgM and IgA isotypes and soluble IL-6 were measured in paired serum and cerebrospinal fluid samples from five patients with neuro-Behçet's syndrome. Another five patients with non-inflammatory neurological diseases were also studied as a control group. Anticardiolipin antibodies, especially the IgM isotype, and IL-6 were highly elevated in the cerebrospinal fluid of patients with neuro-Behçet's. Levels of both IgM isotype anticardiolipin antibodies and IL-6 in the cerebrospinal fluid dropped after disease activity subsided. These results suggest that the increase in IgM isotype anticardiolipin antibodies and IL-6 in cerebrospinal fluid may be involved in the immune response of neuro-Behçet's within the central nervous system. Serial measurements of IgM isotype anticardiolipin antibodies and IL-6 in the cerebrospinal fluid may be useful in evaluating disease activity in neuro-Behçet's.