Clinical, neurophysiologic and laboratory findings obtained in American patients with nervous system Lyme borreliosis were compared to published observations in European neuroborreliosis patients. In both populations, Borrelia burgdorferi infection is commonly associated with neurologic abnormalities. European reports have emphasized dramatic clinical phenomena, such as painful radiculitis (Garin-Bujadoux-Bannwarth syndrome) and chronic progressive spastic paraparesis. North American patients seem to develop milder forms of nervous system involvement. Peripheral nervous system manifestations take a variety of forms, ranging from mild, intermittent sensory symptoms, to typical painful radiculitis. Despite the range of clinical presentations, neurophysiologic and morphologic analyses indicate these all represent different manifestations of the same pathophysiologic process, which, in turn, is similar to what has been described in Garin-Bujadoux-Bannwarth syndrome. Similarly, central nervous system (CNS) symptoms vary widely, ranging from a mild confusional state to a severe encephalitis. The encephalitis is probably due to direct CNS infection. In some instances the confusional state may also be due to CNS infection but it is likely that in many patients it is not. As in European patients, the most reliable indicator of CNS infection appears to be the intrathecal production of anti-B burgdorferi antibodies. Although North American Lyme borreliosis patients may often develop milder forms of nervous system involvement that their European counterparts, there is considerable overlap, and the underlying pathophysiologic mechanisms are probably identical.