Authors retrospectively analyzed the CT findings of cystic tumors of the mediastinum in surgically confirmed 13 cased at Seoul National University Hospital during the recent 3 years from August 1980 to August 1983. The results are as follows : 1. Among 13 cases, bronchogenic cyst and cystic teratoma were 4 cases respectively, pericardial cyst were 2 cases, and thymic cyst and cystic hygroma was 1 case respectively. 2. CT No. of 4 bronchogenic cysts was in the range of 12-112 HU, 3 cases among them showed high density above 69 HU. CT No. of 4 cystic teratomas was in the range of -24 -20 HU. All of them had thick-walled capsules. CT No. of 2 pericardial cysts was in the range of 12-25 HU. CT No. of a thymic cyst was in the range of -10 -23 HU. CT No. of a cystic hygroma was about 24 HU. 3. With CT, cystic tumors of mediastinum are easily diagnosed by its shape, location and typical water density mass. But bronchogenic cyst, because of its mucoid contents, may often have CT No. above those usually considered characteristic of cyst. So we must consider bronchogenic cyst in differential diagnosis even if CT No. is high, if the lesion is in typical location and the shape is round or ovoid.