Malignant transformation of teratoma in the anterior mediastinum is rare; the mass usually has a long history and is seen in older patients. We report a case of teratoma with malignant transformation in the anterior mediastinum, complicated by rupture. CT revealed a lobulated, inhomogeneous cystic mass with a fat component and wall calcifications. The lateral wall was disrupted and consolidation in the adjacent left upper lobe was noted, suggesting rupture. A heterogeneously enhanced solid portion, obliterating the fat plane between the mass and the great vessels was present in the medial aspect of the mass, and pathologic examination demonstrated the presence of adenocarcinoma.