Computed tomography (CT) provides a valuable new perspective in assessing abnormalities of the thorax. In patients with a mediastinal mass or widening detected by plain chest radiography, a definite diagnosis is sometimes possible which would not obtainable by conventional radiological technique. Clinical staging of bronchogenic carcinoma can be achieved by CT better than any other radiologic method. In fifty patients with histologically or angiographically confirmed disease of the thorax, and analysis of chest radiography and chest CT manifestations was made, and the results were as follows: 1. In 27 patients with mediastinal mass detected by chest radiography, a definite diagnosis was possible in 10 patients (36%), who were 6 with teratodermoid, 1 with thymic cyst, 3 with aneurysm. In all patients, the extent and localization of mediastinal mass could be established more precisely than by the chest radiography. 2. In 15 patents with bronchogenic carcinoma, 9 patients (60%) showed hilar adenopathy or mediastinal adenopathy which could not be noted on the chest radiography. 3. Main CT findings of bronchogenic carcinoma were peripheral lung mass, spiculated or lobulated margin, adhesion to pleura or chest wall, and atelectasis of chronic pneumonia. 4. Commonly observed CT findings of teratodermoid were well capsulated mass, calcification, fat density, and multi-loculation. 5. Commonly observed CT findings of thymoma were homogeneous mass, round contour, partially preserved mediastinal fat. CT was superior in evaluation of mediastnum and in the determination of the extent of known bronchogenic carcinoma.