In 14 cases of normal and abnormal thymus proved surgically and histopathologically in Korea Univ. Hae Wha Hospital during recent 6 years, the clinical and CT findings were analysed. 1. Of 14 cases, 2 cases were normal thymus, 5 cases were thymic hyperplasia, 4 cases were benign thymoma, 2 cases were malignant thymoma and 1 case was thymic cyst. 2.Of 14 cases, 10 cases were associated with myasthenia gravis, and 7 of these 10 cases were 3rd to 5the decades females. Among 10 cases with myasthenia gravis, 5 cases were thymic hyperplasia, 1 case was benign thymoma, 2 cases were malignant thymoma, and 2 cases were normal thymus. 3. All 5 thymic hyperplasia were associated with myasthenia gravis. CT findings of thymic hyperplasia were normal in 4 cases and increased lobe thickness in 1 case. 4. Of 4 cases of benign thymoma, only 1 case was associated with myasthenia gravis, and all 4 cases-were positive findings in CT scan. CT findings of benign thymoma were round or oval soft tiss e mass in anterior mediastinum, and 1case had punctate calcifications. 5. Of 2 cases of malignant thymoma, all 2 cases were associated with myasthenia gravis and positive findings in CT scan. CT findings of malignant thymoma were anterior mediastinal soft tissue mass with obliteration of the normal fat planes surrounding great vessels, SVS compression, and pleural tumor implants. 6. CT yielded significant diagnostic information of differential diagnosis between thymoma and thymic hyperplasia in myasthenia gravis patients. Also CT was highly sensitive test in detection of thymoma and determined the extent and invasiveness of thymoma.