The possible association of some viral infections with the onset of Bell's palsy was examined in a study of 29 patients. The results were compared with a sex- and age-matched control group. The number of probable recent viral infections, as judged by a fourfold increase in antibody titers or the presence of specific IgM antibodies, differed statistically from that found in the control group. In seven patients with Bell's palsy the enzyme-linked-immunosorbent assay (ELISA) indicated an acute viral infection (herpes simplex 4; varicella zoster 2; cytomegalovirus 1). All these infections were due to viruses belonging to the herpesvirus group. Clinical evidence of herpesvirus infection was found in three cases (Herpetic eruption). The aetiological relationship between the virological findings and Bell's palsy is discussed. Reactivated herpes simplex virus and transient demyelination of the facial nerve could be one cause of an idiopathic facial palsy.