Nineteen of 283 consecutive patients evaluated in the Multiple Sclerosis (MS) Comprehensive Care Center had a borderline or positive B. burgdorferi serology. In 8 patients a repeat serology was nonconfirmatory. Cerebrospinal fluid was examined in 10 seropositive patients and showed anti-B. burgdorferi antibodies without intrathecal production in 5 patients. Antibiotic treatment did not prevent subsequent neurological relapses. The finding of reactive Lyme serology in an MS patient with no suggestive features of the infection is unlikely to indicate neurological Lyme disease.