Paired serum and cerebrospinal fluid (CSF) specimens from 14 patients with systemic lupus erythematosus (SLE) and central nervous system (CNS) involvement were studied for interleukin-6 (IL-6) activity using the IL-6-dependent murine hybridoma, MH60.BSF2. We also studied 23 patients with noninflammatory neurologic diseases, and 9 SLE patients without CNS involvement. CSF IL-6 activity was elevated only in SLE patients with CNS involvement, although there was no significant difference in serum IL-6 activity among the 3 groups. CSF IL-6 activity was not correlated with either the CSF-serum albumin quotient (Q albumin; an indicator of blood-brain barrier function) or serum IL-6 activity in SLE patients with CNS involvement. The CSF IL-6 activity decreased significantly when CNS manifestations subsided after successful treatment. These results indicate that determination of CSF IL-6 activity may be useful in the evaluation of CNS disease activity in SLE. Moreover, the data confirm the presence of immune system activation within the CNS in patients with SLE-associated CNS disease.