PURPOSE: In order to determine the differential points between them, we analyzed the CT findings of invasivethymoma and thymic carcinoma. MATERIALS AND METHODS: We retrospectively reviewed the CT scans of 14 patients withinvasive 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, directposterior, and anterolateral), obliteration of the fat plane between the mass and vascular structures, vesselencasement, invasion of adjacent mediastinal structures, pleural implants, mediastinal nodes and distantmetastasis. RESULTS: Direct posterior spread was more common in thymic carcinoma than invasive thymoma ; it wasseen in one case (7%) of invasive thymoma and 12(80%) of thymic carcinoma(p=0.00). Posterior spread was seen insix cases (43%) of invasive thymoma and nine (60%) of thymic carcinoma. Anterolateral spread was seen only in twocases (13%) of thymic carcinoma. Obliteration of the fat plane was seen in nine cases (64%) of invasive thymomaand 14 (93%) of thymic carcinoma, while vessel encasement was seen in two cases (14%) of invasive thymoma and13(87%) of thymic carcinoma(p=0.00). Invasion of adjacent structures was seen in two cases (14%) of invasivethymoma and eight (53%) of thymic carcinoma. Pleural implants were more common in invasive thymoma than thymiccarcinoma, being seen in six cases (43%) of the former and one (7%) of the latter(p=0.04). Mediastinallymphadenopathy was seen in three cases (21%) of invasive thymoma and ten (67%) of thymic carcinoma. Distantmetastases were observed only in six cases (40%) of thymic carcinoma(p=0.02). CONCLUSION: Althoughdifferentiation between invasive thymoma and thymic carcinoma is difficult on the basis of CT findings, there arecertain 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 morecommonly associated with pleural implants than were thymic carcinoma.