The relationship between myasthenia grabis and the thymus was well established and myasthenia gravis occurs in the presence of thymic hyperplasia or thymoma or occasionally in histologically normal thymus. Since not every patients with myasthenia gravis is a candidate for thymectomy unless a thymoma is present the differentiation of normal and hyperplastic thymus from thymoma becomes important. Authors reviewed retrospectively clinical records and chest CT scans of 18 patients with myasthenia gravis who underwent thymectomy during recent 5 years to evaluate the role of CT scan. The results were as follows: 1. Of total 18 cases 5 cases had normal thymus 6 cases had thymic hyperplasia 4 cases had benign thymoma and 3 cases had malignant thymoma. 2. Of 5 cases of normal thymus no false positive cases were noted in CT scan. 3. Of 6 cases of thymic hyperplasia CT findings were normal except 1 cases of thickened left thymic lobe. 4. Of 7 cases of thymoma no false negative cases were noted tin CT scan. 5. CT findings of benign thymoma were round or oval shaped discrete slightly enhancing soft tissue mass in anterior mediastinum. 6. CT findings of malignant thymoma were lobulated contoured infiltrative soft tissue mass lesion in anterior mediastinum with calcifications pleural tumor implants and SVC com-pression. CT yielded valuable information on differential diagnosis of thymoma thymic hyperplasia and normal thymus. Also CT was a highly sensitive method in the detection of thymoma and determining the extent and invasiveness