This prospective study reports the clinical and epidemiological features of 187 consecutive patients with neuroborreliosis recognized in Denmark over the 6-yr period, 1985-1990. Only patients with intrathecal Borrelia burgdorferi specific antibody synthesis were included. In 1990 regional incidences varied between 5.7 and 24.1 per million. Ninety-four percent of the patients had early (second stage) neuroborreliosis. The most common manifestation was a painful lymphocytic meningoradiculitis (Bannwarth's syndrome) either with paresis (61%) or as a radicular pain syndrome only (25%). Central nervous system (CNS) involvement in early neuroborreliosis was rare; 4% had signs of myelitis and only one patient had acute encephalitis. Children showed a different course of the disease. Six percent of the patients suffered a chronic course with a disease duration between 6 mths and 6 yrs either as chronic lymphocytic meningitis (1.6%) or as third stage chronic encephalomyelitis (4.3%). Meningeal signs were rare despite pronounced inflammatory cerebrospinal fluid (CSF) changes (median cell count 160/microliters; median protein concentration 1.13 g/l). High dose i.v. penicillin G was administered to 91% of the patients. Based on the clinical outcome and normalization of CSF no treatment failures were recognized. The final morbidity after a median follow-up of 33 mths was low; disabling sequelae were reported in nine patients, mainly those with previous CNS involvement. We conclude that neuroborreliosis is a common and characteristic neurological disorder. The diagnosis should be based on the demonstration of inflammatory CSF changes and B. burgdorferi specific intrathecal antibody production.