A 19-year-old male visited our clinic with the complaint of lower abdominal distention, January 1992. He had undergone living renal transplantation from his mother as a donor on June 15, 1988. But the transplanted kidney had become functional loss by chronic rejection 3 years after transplantation. The computed tomography showed huge low density mass around the non-functioning transplanted kidney. En block resection of the tumor and the transplanted kidney was performed. The tumor was pathologically diagnosed as mixoid liposarcoma. It was proved that the tumor cells were derived from recipient cells by the investigation of HLA-DRB1 DNA typing.