A case of malignant fibrous histiocytoma (MFH) occurring in th retroperitoneum with giant pyonephrosis is reported. The patient was a 45-year-old male and his chief complaint was an abdominal mass. The abdominal fullness progressed so rapidly that he was admitted to our hospital. After examination, this case was diagnosed as a malignant tumor with left hydronephrosis, and an operation was performed on August 5, 1982. At operation, the left kidney contained about 11,000 ml of a pus-like fluid and in the retroperitoneum was found a hen-egg-sized solid tumor which was invading into the left kidney and the feeding vessels of the descending colon. So the tumor, left kidney and a part of the descending colon were resected en bloc. Pathological diagnosis was malignant fibrous histiocytoma. Chemotherapy (PPM regimen) and immunotherapy (OK-432) were administered after the operation, but multiple metastases appeared in the liver and bilateral lungs within 3 months. Then, the CY-VA-DIC regimen was followed. But, local recurrence was found in about 5 months, and the patient died on the 174 th day after the operation. Local recurrence and metastases in the liver, bilateral lungs, pleura and bones were confirmed at autopsy. Besides our case, a review of case reports of retroperitoneal MFH in Japan and comments are presented.