A case of isolated central nervous system involvement in Lyme disease is described. A 13 year-old boy developed progressive spastic quadraparesis, chronic lymphocytic meningitis with a low CSF glucose concentration and demyelinating lesions of the white matter on MRI. The diagnosis was proved serologically by high antibody titers against Borrelia burgdorferi (BB) in the serum (1:5, 120) and CSF (1:1,280). There was evidence of specific intrathecal immune response against the BB antigen. The patient was treated with penicillin G and then ceftriaxone. The CSF abnormalities quickly improved but improvement of the neurologic symptoms was gradual and to date still incomplete.