Dedifferentiated liposarcoma, a subtype of liposarcoma, is characterized by juxtaposition of well-differentiated liposarcoma with a nonadipocytic sarcoma. A peculiar form of dedifferentiated liposarcoma has been described, characterized by a nonlipogenic component with a neural-like whorling pattern of growth and metaplastic bone formation. We report the cytogenetic and molecular genetic findings of this peculiar form of dedifferentiation in a retroperitoneal tumor found in a 58-year-old woman. The neoplasm had the typical histologic findings and a complex karyotype characterized by several numeric and structural chromosome abnormalities, including the presence of ring and giant rod chromosomes. Molecular genetic studies found high levels of amplification of the MDM2 oncogene, consistent with the amplification of the 12q14 chromosome band, a cytogenetic abnormality commonly found in these tumors. These findings indicate that, despite its unique and peculiar morphologic features, this unusual type of dedifferentiated liposarcoma shares many of the cytogenetic and molecular genetic abnormalities found in other forms of dedifferentiation. However, the specific cytogenetic and molecular determinants of these peculiar morphologic findings remain unknown.