Eight years of case histories of patients with positive serum tests for Borrelia burgdorferi (Bb) are reviewed with the aims of analyzing the neurological manifestations involved and of assessing the value of serology in the diagnosis of neuroborreliosis (NB) in our clinical setting. Of the 105 cases with neurological manifestations that could be examined, 25 patients (24%) with other infections, neoplasms or other diseases had false positive serologies. Forty-one patients (39%) met the criteria for NB diagnosis: 26 had lymphocytic meningitis, 13 had polyradiculitis and 20 had cranial neuropathy. Spinal fluid cultures were positive in 43% of the patients on whom the test was performed, and all patients in this group who were treated with antibiotics improved. The other 39 patients (37%) had only neurological manifestations, which were not typical of NB (peripheral neuropathy, stroke, demyelinating disease, dementia or myelopathy), but for which no other etiology could be demonstrated. Spinal fluid serology was positive in 16% of those cases studied and none of those treated only with antibiotics improved. Among patients with both neurological manifestations and Bb positive serology, there were many false positives and cases with signs that were of dubious relation to infection by Bb; therefore, the prevalence of cases consistent with NB is low [corrected].