Thirty cases of endometrial malignancy were successfully studied cytogenetically. Twenty-four of the cases were endometrial adenocarcinomas. Twenty-three of these were stage I tumors, and the twenty-fourth was a stage IV tumor. Fourteen of the stage I adenocarcinomas had abnormal chromosomes, and nine had apparently normal chromosomes. No two tumors had an identical chromosomal rearrangement. Trisomy 1q was common however; 10 of the 14 tumors with abnormal chromosomes had a chromosome 1 abnormality. Clinical and pathologic data were available for 17 of these patients. There appeared to be no relationship between prognostic indicators and the tumor karyotype except for uterine size. Six cases of endometrial malignant mixed müllerian tumor (MMMT) of the homologous type were also analyzed cytogenetically. Three of these tumors had very abnormal karyotypes, and the remaining three had apparently normal chromosomes. Structural abnormalities of chromosome 1, 3, and 5 were present in all three tumors with abnormal karyotypes, but identical breakpoints or rearrangements were not observed.