We studied 38 patients with Lyme meningitis, a newly recognized spirochetal infection. The patients characteristically had intermittent attacks of severe headache, mild meningismus, and a predominantly lymphocytic pleocytosis. In addition to meningitis, 11 patients experienced subtle encephalitic signs, 19 had cranial neuritis, most commonly unilateral or bilateral facial palsy, and 12 developed peripheral radiculoneuritis, plexitis, or mononeuritis multiplex. Without antibiotic therapy, the duration of neurologic involvement was 3 to 18 months. Although sometimes incomplete, the triad of neurologic manifestations of Lyme disease--meningitis, cranial neuritis, and radiculoneuritis--presents a unique clinical picture.