The association of thymoma with myasthenia gravis has been well documented. However, the relationship between these two syndromes and Addison disease are very rarely encountered in clinical practice. We report on a 32-year-old man who underwent a resection for thymoma 48 months ago. The diagnosis of Addison disease was made followed by a diagnosis of myasthenia gravis on the basis of a high titer of acetylcholine receptor levels. The treatment of oral prednisolone 7.5 mg/day and oral prostigmine 180 mg/day was initiated. His symptoms and physical signs were improved after this treatment. To our knowledge, this is the fourth reported case of thymoma synchronously associated with myasthenia gravis and Addison disease.