A 61-year-old woman was admitted for chest discomfort. She had been admitted before, in March 1995, because of a lesion detected on chest roentgenograms. At that time, she was given a diagnosis of mediastinal lipoma based on the findings of chest computed tomography (CT) and magnetic resonance imaging (MRI), but was discharged without active intervention due to lack of subjective symptoms. During follow-up, the patient again reported chest discomfort beginning in March 1998. Because chest radiography disclosed. The tumor had enlarged, the patient was admitted to the hospital by our department. Chest MRI disclosed a mass with a signal intensity equal to that of subcutaneous fat in the pericardial space on both T1-weighted and T2-weighted images. Although sagittal images demonstrated continuity of the mass into intraperitoneal fat, a conclusive diagnosis of diaphragmatic hernia could not be made at that time. On April 30, 1998, a thoracotomy was performed on the basis of a preoperative diagnosis of mediastinal lipoma. During surgery, a hernial ring was observed slightly to the right and behind the sternum. The hernia consisted only of greater omentum, and was diagnosed as Morgagni's foramen hernia.