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 September 1979 to August 1983. 1. Among 37 cases, malignant lymphoma were 7 cases, thymoma and vascular lesion or anomaly were 5 cases respectively, benign teratoma and tuberculous mediastinal lymphadenitis and neurogenic tumor were 4 cases respectively, pericardial cyst were 2 cases, bronchogenic cyst, non-specific cyst, pancreatic pseudocyst, mesothelioma, Bochdal다 hernia was 1 case respectively. 2. The sex ratio between male and female was about 1:1 and the majority of the patients with majority of the patients with malignant lIymphoma and teratoma was under 20 years old. 3. CT findings of the each mediastinal lesion. 1) Primary mediastinal 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 the center of the mass, representing the tumor necrosis. (3) CT was also able to show involvement of other mediastinal lymph 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-shadows were seen along the courses of the bronchi and the vessels. 2) Thyoma (1) A discrete, soft tissue mass was seen in the superior and the anterior mediastinums (mainly posterior to manubrium and anterior to aortic arch) replacing the normal mediastinal fat. (2) In benign thymoma, the marging of the mass was smooth and the normal fat plane between the mass and the vascular structures was well preserved, but invasive thymoma showed obliteration of the normal fat plane, irregular and ragged tumor-lung interface, and irregular thick-ening of pleura and pericardium by tumor implantation to these structures. (3) Teratoma (1) A discrete and smooth marginated 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 charateristic, thick-walled cystic appearances.