Glial fibrillary acidic protein (GFAp), the main protein constituent of the intermediate filaments of astrocytes, was analysed in the cerebrospinal fluid (CSF) of 20 patients with Lyme neuroborreliosis as a marker of the astroglial reaction. The mean GFAp level before antibiotic treatment in the study group was significantly elevated (592 pg/ml +/- 596 [SD]) compared to that in 24 healthy controls (121 +/- 87 [SD]) (p < 0.01). The highest CSF-GFAp levels were seen in the patients with the most severe disease, but the levels were also increased in patients with peripheral paresis, such as facial palsy with no or only minor encephalitic symptoms. This implies that the infection was not limited to radix dorsalis or the meningeal tissues, but affected the central nervous system as well. Furthermore, the astroglial reaction seemed to occur early in Lyme neuroborreliosis since CSF-GFAp levels were elevated also in patients with recent (< 3 weeks) onset of disease. After antibiotic treatment, the GFAp levels decreased. It is suggested the CSF-GFAp concentrations might be useful for monitoring CNS involvement in Lyme neuroborreliosis.