We report the first known patient with human T lymphotropic virus type I (HTLV-I) associated myelopathy (HAM) and myasthenia gravis (MG). A 50-year-old woman developed fluctuating muscle weakness with easy fatigability, transient bilateral blepharoptosis and double vision. Spastic paraparesis complicated these symptoms. Neurological assessments and specific laboratory findings revealed that the patient had definite HAM and MG. By inference from descreasing serum anti-HTLV-I antibody titers after thymectomy, the presence of antigenicity for HTLV-I in the thymic reticular cells, and a high incidence of various coexistent autoimmune diseases in HAM or MG, we suggested the possibility that these two diseases were associated with each other and with HTLV-I infection.