Sera from all 41 adult patients with idiopathic facil paralysis (Bell palsy) and 35 (85%) of 41 matched controls who had never had Bell palsy contained antibodies to herpes simplex virus (P smaller than.05). The frequency of antibodies to herpes zoster virus did not differ in patients and controls. A rise in antibody titer, indicating primary herpes simplex virus infection, was not found in these patients. That Bell palsy may be caused by reactivation of herpes simplex virus is suggested by (1) clinical, neurologic, laboratory, and immunologic similarities between idiopathic facial paralysis and known manifestations of reactivated herpes simplex virus infection, and (2) the known neurotropism of herpes simplex virus, including its presence in latent form in the trigeminal ganglia, and parallels with known facial paralysis due to varicella zoster virus, a closely related agent. The presence of antibodies to herpes simplex virus is the only common factor among the patients tested in this study.