Authors retrospectively analized the CT findings of mediastinal lesions in surgically or clinically confirmed 37 cases at kosin Medical College during the recent 4 years from Sept. 1979 to Aug. 1983. 1. Among 37 cases,malignant lymphoma were 7 cases, thymoma and vascular lesion or anomaly were 5 cases respectively, benign teratomaand tuberculous mediastinal lymphadentis and neurogenic tumor were 4 cases respectively. pericardial cyst were 2 cases, bronchogenic cyst, non-specific cyst, pancreatic pseudocyst, mesothelioma, Bochdalek hernia was 1 caserespectively. 2. The sex ratio between male and female was about 1:1 and the majority of the patients withmalignant lymphoma and teratoma was under 20 years old. 3. CT findings of the each mediastinal lesion. 1) Primarymediastinal malignant lymphoma. (1) A large, matted, continuous and midline-crossing mass was observed in the superior and the anterior mediastinums in all cases. (2) In 3 cases, irregular lower densities were seen in thecenter of the mass, representing the tumor necrosis. (3) CT was also able to show invovlement of other mediastinallymph nodes and adjacent structures such as pleura, anterior chest wall and lung parenchyma. (4) Involving pleura,homogeneous band-like shadows were seen along the pleura, appearing denser than the associated pleural effusion.(5) Involving lung parenchyma, irregular-shaped nodules and band-llike densities were seen along the courses ofthe bronchi and the vessels. 2) Thyoma. (1) A discrete, soft tissue mass was seen in the superior and the anteriormediastinums(mainly posterior to manubrium and anterior to aortic arch) replacing the normal mediastinal fat. (2)In benign thymoma, the margin of the mass was smooth and the normal fat plane between the mass and the vascularstructures was well preserved, but invasive thymoma showed obliteration of the normal fat plane, irregular andragged tumor-lung interface, and irregular thickening of pleura and pericardium by tumor implantation to thesestructures. (3) The tumors spreaded to only one side of the chest cavity. 3) Teratoma (1) A discrete and smoothmarginated mass was seen in anterior mediastinum and it has three or more of different tissue densities among fat,water, soft tissue and calcific densities. (2) 3 cases of teratomas showed the characteristic, thick-walled cysticappearances.