The incidence of myasthenia gravis in patients with thymomas has been reported from 10 to 50 percent, and the controversy ranges from 8-15 percent. This is the report of myasthenia grayis combined with benign thymoma and obtained relatively good result through surgical treatment at the department of thoracic surgery, National Medical center. The thymoma was mixed type and benign nature. Post operatively this patient was developed myasthenia crisis, and treated with mechanical respirator, and recurred the symptoms about 2months later, but well controlled with medical treatment.[KTCS 1974;1:123-126]