Sixty-two patients with acute idiopathic peripheral facial nerve palsy (AIPFP) and 31 patients with lymphocytic meningoradiculitis (Garin-Bujadoux or Bannwarth's syndrome) are described. Results of cerebrospinal fluid (CSF) analysis, including the measurement of immunoglobulins (Ig) G, A, and M, indicate that pleocytosis and/or disturbance of the blood-CSF barrier (BCB) and/or local immunoglobulin synthesis within the central nervous system (CNS) do occur in about 25% of patients with AIPFP. The commonest finding is a slight to moderate breakdown of BCB function without evidence of intrathecal immunoglobulin synthesis. In only about 10% of patients, further support for an inflammatory process within the CNS is found by intrathecal synthesis of oligoclonal IgG and/or localized synthesis of IgG and/or IgA. The majority of cases (75%) do not show any signs of an inflammatory process within the CNS. In contrast, lymphocytic meningopolyradiculitis (LMR) has a characteristic CSF profile with early impairment of BCB permeability as well as with rapid and predominant intrathecal IgM synthesis, which helps to distinguish monosymptomatic LMR from AIPFP. By applying a sensitive enzyme-linked immunosorbent assay to identical concentrations of IgG in serum and CSF, evidence of intrathecal synthesis of virus-specific antibodies was found only in 2 of 13 patients with AIPFP. In contrast, all 4 patients with herpes zoster oticus and peripheral facial palsy (Ramsay Hunt syndrome) showed an intrathecal IgG synthesis to varicella zoster virus lasting for up to 4 months after onset of disease.