Three of 6 patients with spastic paraparesis in Lima, Peru, were found to have antibodies to human T‐lymphotropic virus type I (HTLV‐I). Blood and cerebrospinal fluid antibodies were confirmed by Western blot analysis. Multilobulated lymphocytes in blood and cerebrospinal fluid of the index case stained with monoclonal antibodies for T‐helper cells and for T10, an activation marker. Blood mononuclear cells from patients with HTLV‐I–associated myelopathy showed spontaneous proliferation in culture, evidence of interleukin‐2 receptors, and decreased natural killer cell activity. Copyright © 1988 American Neurological Association