A 3-1/2-year-old girl was diagnosed as having a mediastinal endodermal sinus tumor with pulmonary, bony, and hilar lymph node metastases. Following 7 months of treatment with chemotherapy, thoracic CT (computerized tomogram) scan showed the presence of a residual mass. Upon further surgical exploration the residual mass, which was found to be intrapericardial and arising from the ascending aorta, was resected. This second lesion was shown to be a benign cystic teratoma. The combination of an intrapericardial teratoma and a mediastinal endodermal sinus tumor in a female child is probably unique. The possible etiology for the combination of these two pathologies is discussed.