Abstract Cytogenetic analyses of four malignant mixed mesodermal tumors (MMMT) of the uterus are reported, of which one was of the homologous type and three of the heterologous type. Karyotypic analyses were obtained in two cases from original tumors and in two cases from tumors xenotransplanted into nude mice. The karyotype of the homologous MMMT was normal in three different passages of a nude mice xenograft line established from the primary tumor. The heterologous tumors showed normal karyotype in one case and hyperdiploid and near triploid range with extensive numerical and structural rearrangements in two cases. Deletion of chromosome 1 at p32, and deletion of chromosome 11 at q13 were common markers in anomalous cases. The chromosomes most often involved in structural rearrangements were chromosomes 1, 9, 11, 12, 17, and 19. Double minutes, homogeneously staining regions, and telomeric association were also seen.