Myasthenia gravis, a disease characterized by weakness and easy fatigue of skeletal muscles, has been associated with other diseases of presumed autoimmune nature. These include rheumatoid arthritis, systemic lupus erythematosus, thyroid dysfunction, hematologic disease, gammopathy, infection, cancer, inflammatory myopathy, etc. In some cases of myasthenia gravis, an inflammatory myopathy develops and adds to the weakness already caused by the transmission defect. We report a 31-year-old female who had the manifestations of myasthenia gravis and polymyositis with the brief review of literatures.