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A woman in her late 70s presented with progressive limb weakness, muscle atrophy and hyper-reflexia. Laboratory findings revealed elevated creatine kinase and positive serum human T-cell leukaemia virus type 1 (HTLV-1) antibody. Clinical and electrophysiological findings met revised El Escorial criteria for amyotrophic lateral sclerosis (ALS), but muscle MRI showed inflammatory changes. Muscle biopsy revealed both neurogenic and inflammatory features. While methylprednisolone showed no benefit, intravenous immunoglobulin therapy produced transient improvement in weakness with normalisation of creatine kinase levels. The patient died from respiratory failure 3 years after symptom onset. Autopsy confirmed typical ALS-TDP pathology with phosphorylated TDP-43 inclusions in motor neurons. HTLV-1 Tax-positive lymphocytes infiltrated skeletal muscles but not the central nervous system, establishing dual pathology of ALS-TDP with HTLV-1-associated myositis. The improvement most likely reflected treatment of the HTLV-1-associated myositis rather than the underlying motor neuron disease. This case highlights the importance of evaluating treatable conditions in HTLV-1-seropositive ALS patients.