CT provides a valuable new perspective in assessing abnormalities of the thorax. In patients with amediastinal mass or widening detected by plain chest radiography, a definite diagnosis is sometimes possible whichwould not obtainable by conventional radiological technique. Clinical staging of bronchogenic carcinoma can beachieved by CT better than any other radiologic method. In fifty patients with histologically or angiographically confirmed disease of the thorax,an analysis of chest radiography and chest CT manifestations was made, and theresults were as follows; 1. 27 patients with mediastinal mass detected by chest radiography, a definite diagnosiswas possible in 10 patients (36%), who were 6 with teratodermoid, 1 with thymic cyst, 3 with aneurysm. In allpatients, the extent and localization of mediastinal mass could be established more precisely than by the chestradiography. 2. In 15 patients wtih bronchogenic carcinoma, 9 patients (60%) showed hilar adenopathy ormediastinal adenopathy which could not be noted on the chest radiography. 3. Main CT findings of braonchogeniccarcinoma were peripheral lung mass, spiculated or lobulated margin, adhesion to pleura or chest wall, andatelectasis or 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 homogenousmass, round contour, partially preserved mediastinal fat. CT was superior in evaluation of mediastinum and in the detemination of the extent of known bronchogenic carcinoma.