We report a Swedish patient with progressive dementia possibly associated with human T cell-lymphotropic virus type I (HTLV-I) infection. The clinical investigation revealed no typical sings of other neurological disorders. The patient was probably infected in East-Asia 35 years before onset of the disease. High titers of specific HTLV-I antibodies were detectable with solid-phase peptide ELISA in serum (1:1.600) and cerebrospinal fluid (CSF) (1:20), and the CSF/serum anti-HTLV-I antibody ratio indicated intrathecal HTLV-I antibody synthesis. Western blot for HTLV-I and polymerase chain reaction with primers selected for the HTLV-I pol gene were positive in both peripheral blood and cerebrospinal fluid. HTLV-I antigen was also demonstrated after in vitro co-cultivation of mononuclear cells from peripheral blood. Thus, our findings indicate that HTLV-I infection also may be associated with dementia. In addition, this case report calls attention upon HTLV-I as a possible etiologic agent to neurological diseases in countries previously spared from the infection.