In 14 cases of normal and abnormal thymus proved surgically and histopathologically in Korea University Hae Wha Hospital during recent 6 years, the clinal and CT findings were analysed. 1. Of 14 cases, 2 ases were normalthymus, 5 cases were thymic hyperplasia, 4 cases were benign thymoma, 2 cases were malignant thymoma and 1 casewas thymic cyst. 2. Of 14 cases, 10 cases were associated with myasthenia gravis, and 7 of these 10 cases were 3rdto 5th decades females. Among 10 cases with myasthenia gravis, 5 cases were thymic hyperplasia, 1 case was benignthymoma, 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 lobethickness in 1 case. 4. Of 4 cases of benign thymoma, only 1 case was associated with myasthenia gravis, and all 4cases-were positive findings in CT scan. CT findings of benign thymoma were round or oval soft tissue mass inanterior mediastinum, and 1 case had punctate calcifications. 5. Of 2 cases of malignant thymoma, all 2 cases were associated with myasthenia gravis and postive 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, SVC 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 testin detection of thymoma and determined the extent and invasiveness of thymoma.