Myositis is one of well-known complications of HTLV-1 associated myelopathy (HAM). On twelve HAM patients, 10 women and 2 men complicated with myositis, we examined their clinical symptoms, muscle pathology, and therapeutic response. Clinical examination revealed gait disturbance in 10, weakness of upper limbs in 6, muscle atrophy in 4, and myalgia in 3 patients. Blood test disclosed elevated serum CK level in 6 patients. Muscle biopsy showed marked inflammatory changes with many necrotic and degenerating fibers in 7 patients, and the rest showed only focal invasion of inflammatory cells. Eight patients received corticosteroid or immunosuppressant therapy. The neurological symptoms were improved in 5 of 8 patients 3 months after the treatment. And only one out of 5 patients whom we followed up 3 years after the treatment showed improvement.