Of the 96 serologically confirmed neuroborreliosis cases seen in our clinic between 1983 and 1988, 11 patients had mild to moderate and 4 patients had serious cerebral and/or spinal cord symptoms. Nine of these 15 patients with CNS involvement exhibited a primary chronic course of the illness. After high-dose intravenous therapy with penicillin, doxycycline or cefotaxime, given mostly in combination with cortisone, gradual recovery occurred with normalization of CSF findings characteristic of neuroborreliosis, and normalization of significantly elevated Borrelia burgdorferi IgG antibody titres in CSF and serum. Brain MRI and CT showed evidence of or were suggestive of vascular involvement which correlated with clinical symptoms in 11 of the 15 patients with CNS involvement. Brain MRI changes that were similar but much slighter in number and intensity were seen in 5 of 12 neuroborreliosis patients without clinical signs of CNS involvement (lymphocytic meningoradiculitis; Bannwarth's syndrome). The frequencies of the HLA-DR7 (75%), HLA-B44 (50%) and HLA-A29 (33%) antigens in 12 neuroborreliosis patients with clinical symptoms of CNS involvement were significantly different from the frequencies in 12 neuroborreliosis patients without CNS involvement and in 100 control subjects. Diagnostic criteria of active neuroborreliosis are proposed.