We present a case of subacute meningoencephalitis associated with human T-lymphotrophic virus Type I (HTLV-I) infection, for HTLV-I antibody being positive in serum and cerebrospinal fluid. Necropsy findings disclosed leptomeningeal and parenchymal mononuclear cell infiltration with multinucleate giant cells, which are similar to those seen in subacute encephalitis with acquired immunodeficiency syndrome (AIDS).