Purpose : In order to determine the differential points between them, we analyzed the CT findings of invasive thymoma and thymic carcinoma. Materials and Methods : We retrospectively reviewed the CT scans of 14 patients with invasive thymoma and 15 with thymic carcinoma, confirmed by surgery(n=19) or percutaneous needle aspiration (n=10)between 1988 and 1996. CT findings were evaluated in each group for intrathoracic spread (posterior, direct posterior, and anterolateral), obliteration of the fat plane between the mass and vascular structures, vessel encasement, invasion of adjacent mediastinal structures, pleural implants, mediastinal nodes and distant metastasis. Results : Direct posterior spread was more common in thymic carcinoma than invasive thymoma ; it was seen in one case (7%) of invasive thymoma and 12(80%) of thymic carcinoma(p=0.00). Posterior spread was seen in six cases (43%) of invasive thymoma and nine (60%) of thymic carcinoma. Anterolateral spread was seen only in two cases (13%) of thymic carcinoma. Obliteration of the fat plane was seen in nine cases (64%) of invasive thymoma and 14 (93%) of thymic carcinoma, while vessel encasement was seen in two cases (14%) of invasive thymoma and 13(87%) of thymic carcinoma(p=0.00). Invasion of adjacent structures was seen in two cases (14%) of invasive thymoma and eight (53%) of thymic carcinoma. Pleural implants were more common in invasive thymoma than thymic carcinoma, being seen in six cases (43%) of the former and one (7%) of the latter(p=0.04). Mediastinal lymphadenopathy was seen in three cases (21%) of invasive thymoma and ten (67%) of thymic carcinoma. Distant metastases were observed only in six cases (40%) of thymic carcinoma(p=0.02). Conclusion : Although differentiation between invasive thymoma and thymic carcinoma is difficult on the basis of CT findings, there are certain differential points. Thymic carcinomas showed a higher rate of direct posterior intrathoracic spread, vessel encasement, mediastinal nodes and distant metastases than invasive thymomas. These, however, were more commonly associated with pleural implants than were thymic carcinoma.