To assess the association of the human T-cell lymphotropic virus type I (HTLV-I) and idiopathic facial nerve palsy of the lower motor neuron type, we studied 78 patients consecutively admitted to the Port of Spain General Hospital in Trinidad, the West Indies, with a confirmed diagnosis of idiopathic facial nerve palsy. Patients were compared with two control groups: a population-based group of persons 20 years and older and a hospital-based group of patients 15 to 84 years old admitted to the medical wards. Sixty-two patients were Trinidadians of African origin and 16 were Trinidadians of East Indian origin. None of the East Indian patients was HTLV-I antibody positive. Three Afro-Trinidadians were infected with human immunodeficiency virus type 1 and 1 was coinfected with this virus and HTLV-I. Of the remaining 58 Afro-Trinidadians, 12 (20.7%) were HTLV-I positive only. This rate was statistically higher than the HTLV-I seroprevalence in the Afro-Trinidadian general population (3.5%) and the hospital control group (5.6%). After age standardization, the HTLV-I prevalence for patients with facial nerve palsy remained significantly elevated. HTLV-I antibody assays should be performed on all patients with idiopathic facial nerve palsy of the lower motor neuron type who live in HTLV-I endemic areas or are migrants from these areas.