Human T-lymphotropic virus type I (HTLV-I), the etiological agent of adult T-cell leukemia/lymphoma, also appears to be the cause of tropical spastic paraparesis, a chronic myelopathy reported in several different regions of the world. The prevalence of antibodies to HTLV-I in patients with chronic neurodegenerative disorders other than tropical spastic paraparesis and in patients with some muscle inflammatory disorders has been investigated. IgG antibodies to HTLV-I were measured in the sera and/or cerebrospinal fluid from 82 Guamanian patients with amyotrophic lateral sclerosis and parkinsonism-dementia, 164 Guamanian normal controls, 10 patients with kuru from the Eastern Highlands of Papua New Guinea, 4 patients with Viliuisk encephalomyelitis from the Iakut region of eastern Siberia, 45 Italian patients with multiple sclerosis, and 56 patients with polymyositis (49 from the United States and 7 from Jamaica). As determined by enzyme-linked immunosorbent assay, Western immunoblot, and gelatin particle agglutination techniques, serological evidence of HTLV-I infection was found in 1 patient with amyotrophic lateral sclerosis and 1 control subject from Guam, and in 1 patient from the United States and all 7 Jamaican patients with polymyositis. Except for the high seropositivity rate among the group of Jamaican patients with polymyositis, our data indicate that HTLV-I is an unlikely causative agent in the spectrum of the neurological diseases examined. The seropositivity of the 7 Jamaican patients with polymyositis requires further study.