Patients with human immunodeficiency virus (HIV) have often progressive dementia. Human T cell lymphotropic virus Type I (HTLV-I) infection has not been reported to cause dementia. We tested antibodies to HTLV-I and HIV in serum and cerebrospinal fluid in 69 Finnish patients referred because of dementia to an outpatient department of neurology. No antibodies to HTLV-I and HIV were detected in patients with the clinical diagnosis of Alzheimer's disease, vascular dementia, secondary dementia due to a specific cause, or in cases of atypical dementia.