A 47-year-old woman was admitted to the Nagoya City University Hospital, Nagoya, Japan, complaining of an abdominal swelling that had been present for the past 12 months. Her abdomen was markedly swollen and felt hard without tenderness. Peripheral blood examination revealed slight anemia and slight elevation of C-reactive protein. No other disorders, including any tumor markers, were detected. A computed tomography scan (CT) showed two masses in the retroperitoneum, one of which was 26 x 15 x 29 cm and enhanced slightly, and the right kidney was involved and pushed aside. Almost all retroperitoneal cavities were filled with the giant tumor. The other mass was walnut-sized tumor adjacent to the large one, and mostly of low density. Magnetic resonance imaging (MRI) revealed the giant tumor was T1-low intensity and T2-high intensity, but the small tumor was T1-high intensity, with a changed low signal in fat suppression (Figure 1). This indicated that the small tumor was mainly fat-containing, and the giant tumor appeared to be mainly composed of mucinous tissue. Under general anesthesia, the tumor was excised with the right kidney en bloc. There was not much adhesion to surrounding tissues. Upon gross examination, the tumor was well circumscribed and encapsulated and weighed 8.5 kg (Figure 2). The tumor had two components as determined by CT and MRI. The two components were very clearly divided. The small one was yellowish, and histopathologically, they were mainly composed of fat cells that varied in size, that is, mature-appearing adipose tissue with scattered lipoblasts exhibiting large hyperchromatic nuclei. Immunohistochemical analysis revealed that the cells were strongly stained by S-100 protein which is a marker for fat (Figure 3). Therefore, this small tumor was diagnosed as a well-differentiated liposarcoma. On the other hand, giant tumor was white and, histopathologically, mainly contained abundant mucinous mesenchyme and filled with atypical spindle cells. There was extensive in-depth invasion into the renal parenchyma, but the surgical margin was negative. According to immunohistochemical analysis, the poorly differentiated tumor cells had no expression of several mesenchymal markers containing S-100 protein. Therefore, a comorbid malignant fibrous histiocytoma (MFH), or other myxoid tumor could not be ruled out. Consequently, we focused attention on FISH analysis, using specific probes for the MDM2 gene (Figure 3). The amplification of MDM2 gene was detected in the nuclei of both small and giant tumors. Thus, a definitive diagnosis was made of the two tumors as a dedifferentiated liposarcoma. The patient was strictly observed without adjuvant therapy. During the last follow-up investigation, performed 12 months after surgical intervention, the patientbecame well with no clinical or radiological signs of recurrence.