In 6 cases of invasive thymoma proved histologically from 1981 to 1984 in Yonsei University Medical Center, the CT findings and pattern were analysed. The results were as follows 1. Of 6 cases, 4 were males and 2 were females. All cases were between 40-64 years and the average was 51 year lod. 2. Of 6 cases, 2 female patients were associated with myasthenia gravis. 3. By the histological examination, 2 were confirmed as mixed cell type, 2 spindle cell type, 1 lymphocytic type and 1 epithelial cell type. 4. CT findings of invasive thymoma were 1) A discrete but lobulated and irregular marginated soft tissue mass in the superoanterior mediastinum repalcing the normal mediastinal fat tissue. 2) Usually irregular low density areas within the mass suggesting central necrosis or calcification in 1 of 6 cases was noted. 3) Local invasiveness of the mass shown as obliteration of the normal far planes surrounding great vessels, irregular thickenings or nodular shadows of the pleura, di gphragm and pericardium and irregular and ragged tumor-lung interfaces if the tumor invaded to these structures. 4) Frequent extention of tumor to middle and post, mediastnum along pericardium or mediastinal pleura with resultant extrinsic indentation and/or invasion of the hilar region. 5) Extensive tumor infiltration to middle and post, mediastinum in 1 cases, indistinguishable from lymphoma. 6) Low attenuation numbered area of brain in another 1 case, but not confimed histologically.