We reported a family case of HAM and HTLV-I carrier including two sisters presenting with myositis. Both the 65-year-old elder sister and her 53-year-old younger sister initially noted lumbago and developed difficulty in walking several years later. Neurologic examination revealed muscle weakness of the proximal parts of the upper and lower limbs (especially the latter). Mild hyperreflexia in all extremities, urinary disturbance and constipation were detected in the elder sister, while only urinary disturbance and constipation were seen in the younger. Anti-HTLV-I antibody in both serum and CSF was positive in both sisters. Muscle biopsy specimens showed features of myositis with perivascular, perimysial and endomysial lymphocyte infiltration. Although an association between HTLV-I and polymyositis had not yet been established, we think that our cases support such an association.