Glial fibrillary acidic protein (GFAp) was analysed in cerebrospinal fluid (CSF) of patients with multiple sclerosis (MS) and healthy controls. Patients with relapsing-remitting course (n = 13) were followed with quantitative neurological examinations and lumbar punctures during a 24-month period. The patient group was a subsample from a randomised, double-blind clinical trial of acyclovir on MS: 7 patients were treated with acyclovir and 6 were placebo controls. CSF was also collected from 5 age-matched healthy individuals with normal quantitative neurological examinations. The CSF assays disclosed increased concentrations of GFAp in MS patients compared to controls (p < 0.01). Furthermore, the GFAp levels correlated significantly with the deficit score (p < 0.01) but not with exacerbation frequency. When the group treated with acyclovir was compared with the placebo group, no significant change of CSF GFAp was observed. In the present study we show that GFAp is increased in CSF of patients with MS and that the levels correlate with the neurological dysfunction. Further work is needed to ascertain whether determinations of CSF GFAp can be used to monitor disease progression in MS or whether the assay may be useful to evaluate therapeutic intervention.