The ability of CT sanning of the mediasinum to distinguish specific tissue densitie and to display in atransverse plane often provides unique diagnsotic information unobtainable with conventional radiographic methods.We retrospectively analyzed the CT findings of 20 cases of proven mediastinal masses at the Departement ofRadiology, St. Mary's Hospital, Catholic Medical College from Feb. 1982 to June 1984. CT scans were performed witha Siemens Somatom 2 scanner. The technical factors involved were tube voltage 125 kVp, exposure time 5 seconds,230 mAs, 256x256 matrices, and pixel size 1.3mm. 8mm slices were obtained at 1cm interval or magnifying scans wereobtained. After pre-contrast scans, contrast scans were routinely taken with rapid drip-infusion of contrastmedia(60% Conray, 150cc). The results obtained were as follows; 1. Among 20 cases, 11 were tumors, 4 infectiousmasses and 5 aneurysms of great vessels, tortuous brachicephalic artery and pericardial fat pad. In each case CTshowed accurate location, extent, and nature of the masses. 2. Solid tumors were thymic hyperplasias, thymoma,thymus carcinoid, neurilemmoma and germ cell tumors(seminoma, embryonal cell carcinoma). Internal architecture washomogeneous in thymoma, thymus carcinomoid, neurilemmoma, seminoma but inhomogeneous in thymic hyperplasias andembryonal cell carcinoma. CT number ranged from 16 to 49 HU and were variably enhanced. 3. Cystic tumors consistedof teratomas, cystic hygroma, and neurilemmoma. Teratomas contained calcium and fat, inhomogeneous mass withstrongly enhancing wall. Cystic hygroma was nonenhancing mass with HU of 20. 4. All of germ cell tumors (2teratomas and one each of seminoma and embryonal cell carcinoma) and one of 2 thymic hyperplasias had calciumdeposit. 5. Tuberculous lymphadenopapthies presented as a mass in the retrocaval pretracheal space and hilarregion with HU ranging from 34 to 64 and enhancement was variable. Mediastinal abscess was alsmost not enhancing,presenting as an inhomogeneous mass of 21 HU with mottled air densities. 6. Aneurysm of great vessels, tortuouobrachiocephalic artery and pericardial fat pad which were often misinterpretted as mediastinal tumor on theconventional radiographic study could be easily distinguishable on CT scan.