Here, we report on an elderly woman with sarcoidosis and Hashimoto's disease who later developed myasthenia gravis. She was 68-year-old with a long history of Hashimoto's disease who had a clinical diagnosis of sarcoidosis with uveritis at the age of 66 years. On laboratory examination, angiotensin-converting enzyme, lysozyme and gamma-globulin were elevated and there was bilateral hilar lymphoadenopathy. She was admitted to our hospital because of left blepharoptosis and mild fatigability in the proximal muscles at the age of 68 years. Myasthenia gravis, type IIa, was confirmed by elevated titer of anti-acetylcholine receptor antibody in serum (0.8 nmol/l, normal < 0.6), positive edrophonium test and decremental EMG response. Oral prednisolone therapy was effective. Her muscle biopsy revealed HLA ABC-positive fibers in all fascicles, and HLA-DR positive fibers in the perifascicular areas. Myasthenia gravis complicated by sarcoidosis and Hashimoto's thyroiditis is extremely rare, suggesting that the common underlying immunological abnormalities for the three disorders such as a certain defective cellular immunity are responsible for the pathomechanism to induce the patient condition.