Unique genetical gain of 8q and/or polysomy 8 identified after short-term culture are presented in three cases of malignant mixed Müllerian tumors (MMMT) of the uterus. All tumors revealed abnormal karyotypes and modal chromosome numbers in the hyperdiploid range. Two out of three cases exclusively demonstrated numerical aberrations, most prominently polysomy 8. One tumor revealed tetrasomy 8 as the sole aberration. The third tumor showed a more complex karyotype, including two unbalanced translocations, leading to partial gain of 8q. These cytogenetic findings, together with 14 previously reported cases of MMMT and 5 cell lines, suggest a distinct subgroup of endometrial MMMT characterized by gain of chromosome 8 or 8q.