Anticardiolipin (ACL) antibodies and interleukin-6 (IL-6) in cerebrospinal fluid (CSF) may be involved in the mechanism of lupus patients with central nervous system (CNS) involvement. ACL antibodies of 3 isotypes and IL-6 were measured in paired CSF and serum samples from 14 lupus patients with CNS involvement, 5 lupus patients without CNS involvement and 7 patients with non-inflammatory neurological diseases. ACL antibodies, IgG and IgM isotypes, and IL-6 were significantly increased in CSF from lupus patients with CNS involvement as compared with other 2 groups of patients. Both ACL antibodies and IL-6 decreased after neurological activity subsided. These results suggest increased ACL antibodies and IL-6 in CSF are involved in immune responses within CNS in lupus patients. Quantitation of CSF ACL antibodies may be helpful in evaluating neurological activity of lupus patients with CNS involvement.