The relationship between myasthenia gravis and the thymus was well established and myasthenia gravis occurs inthe 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 ofnormal and hyperplastic thymus from thymoma becomes important. Authors reviewed retrospectively clinical recordsand 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, 6cases had thymic hyperplasia, 4 cases had benign thymoma and 3 cases had malignant thymoma. 2. Of 5 cases ofnormal thymus, no false positive cases were noted in CT scan. 3. Of 6 cases of thymic hyperplasia, CT findings were normal except 1 case of thickened left thymic lobe. 4. Of 7 cases of thymoma, no false negative cases were noted in 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 anteior mediastinum with calcifications, pleural tumor implants, and SVC compression.CT yielded valuable information on differential diagnosis of thymoma, thymic hyperplasia and normal thymus. AlsoCT was a highly sensitive method in the detection of thymoma and determining the extent and invasiveness.