A CT scan can make accurate diagnoses of most thymic masses by assessing their size, shape, and internal architecture and is especially effective in detecting pleural implants, mediastinal involvement, and pulmonary parenchymal invasion in malignant thymoma. The authors analyzed the CT findings of 10 histologically-proven thymic masses from 1983 to 1990 in Yeungnam University Hospital. There were 10 cases of thymic masses in the anterior mediastinum consisting of 6 benign, 3 invasive thymomas, and one thymolipoma, while myasthenia gravis was associated with 2 cases of benign thymomas and with one case of invasive thymomas. The CT findings of the benign thymomas(6 cases) were well defined, bordered, round-or oval-shaped masses with a well-preserved fat plane between the thymic mass and mediastinal great vessels, with no evidence of pleural implants and lung parenchymal invasion. The CT findings of the invasive thymomas(3 cases) were irregular, marginated lobular masses with obliteration of the fat plane between the thymic mass and surroundings great vessels, with evidence of local invasion such as extension to A-P window and mass effect to bronchus. Irregular pleural thickening due to pleural metastasis, multiple metastatic lung parenchymal nodules, and multiple mediastinal lymph node enlargement were also seen in the invasive thymomas. One case of thymolipoma showed an approximately 20 cm-sized, well-defined fat density mass containing internal septations.