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Anterior horn cell degeneration has only occasionally been noted in patients with tropical spastic paraparesis associated with human T lymphotropic virus type-1 (HTLV-1) infection. We report on three adult patients with HTLV-1-associated polymyositis who had clinical evidence of anterior horn cell degeneration. One patient had moderate proximal weakness and muscle wasting in all four limbs, while two had mild upper limb weakness with more profound proximal weakness and wasting in the lower limbs. In all three patients, electromyographic findings were compatible with motor unit loss and muscle biopsies showed mononuclear inflammatory cell infiltration; muscle biopsies in two patients showed features of denervation. Immunoglobulin G (IgG) antibodies to HTLV-1 were detected by enzyme-linked immunosorbent assay (ELISA) and confirmed by Western immunoblot in serum and cerebrospinal fluid in all three patients. In two, cell cultures were established from peripheral blood lymphocytes and HTLV-1 antigen was identified by immunofluorescence and the ELISA antigen-capture technique using an anti-p19 HTLV-1 mouse monoclonal antibody. The three cases illustrate the variety of neuromuscular disease, other than spastic paraparesis, that may occur in HTLV-1 infection. In some cases of HTLV-1-associated polymyositis, anterior horn cell degeneration may make a significant contribution to the muscle atrophy observed.