Computerized Tomography can make accurate diagnosis in most of the mediastinal tumors and cysts by assessingtheir location, shape and internal architecture. Authors analysed and present CT findings of 89 surgically provenmediastinal tumpors and cysts that were studied and treated in Seoul National University Hospital during recent 5years. The results are as follows; 1. The most common tumor was teratoma(25 cases). Neurogenic tumor (20 cases),thymic tumor or cyst (16 cases), lymphoma(7 cases), bronchogenic cyst (6 cases), intrathoracic goiter (6 cases),pericardial cyst(3 cases) and cystic hygroma(2 cases) were next in order of frequency. 2. The most constant findings of teratoma was thick walled cystic area(100%), while pathognomonic fat and calcified density were seenonly in 52% and 48% of cases, respectively. 22 cases were located in anterior mediastinum, 2 cases were inposterior mediastinum and a case is in middle mediastinum. 3. There were 20 cases of neurogenic tumor consiting of6 neurilemmomas, 7 ganglioneuromas, 4 neurofibromas, 1 ganglioneuroblastoma, 1 neuroblastoma and 1 malignantschwannoma. Most of them were located in posterior mediastinum with exception of 2 neurilemmomas arising from leftvagus nerve and left recurrent laryngeal nerve in middle mediastinum. Cystic change was seen in 2 cases ofneurilemmoma and in a case of ganglioneuroma. Calcification was seen in 3 cases, of neuroblastoma, aneurilemmoma,and a ganglioneuroma. 4. There were 11 cases of thymoma showing homogeneous solid mass with speckeldcalcification in 4 cases and irregular cystic change in 3 cases. 2 cases were invasive thymoma and myastheniagravis was present in 4 cases. A case of thymolipoma and a case of thymic cyst were included. 5. Lymphoma(2Hodgkin's and 4 non-Hodgkin's) appeared as lobulated, matted mass in anterior mediastinum especially inprevascular area expnading bilaterally. 6. Intrathoracic goiter appeared as slingtly high density mass within termingled calcification and cystic area in 5 cases of secondary goiter and homogeneous high densitymass(100-110 H.U. in precontrast scan) in a case of primary goiter. 7. Among the 6 bronchogenic cysts, 3 werelocated in subcarinal area, 2 were above carina and one was in left hilar area. 3 cases showed high CT number morethan 70 H.U. and others showed water density. 8. 3 cases of pericardial cyst were located in right cardiophrenicangle and all of them showed water density. 9. 2 cases of cystic hygroma were located in superior mediastinum,with extension to lower neck and all of them showed water density.