Seven cases of endometrial adenocarcinoma (EC) are reported. Two of these cases exhibited diploid chromosome ranges and showed simple rearrangements involving a chromosomal abnormality of chromosome 10. In four cases, the chromosome number ranged between 50 and 70; rearrangements were more complex, with many abnormalities such as homogeneously stained regions, minutes, dicentrics, and ring chromosomes. In one case, two subpopulations of cells were detected, one in a diploid chromosome range with chromosome 10 altered, and the second, very pleomorphic. These abnormalities are probably due to the evolution of a destabilized genome and represent a consequence of the advanced stage of the disease. The importance of simple abnormalities as clues to the primary chromosomal change, and the possibility that chromosome 10 represents the primary chromosomal alteration event in EC, are discussed.