A loss of the DNA copy number at chromosomal region 11q23-24 as detected by comparative genomic hybridization (CGH) is a marker of poor prognosis in patients with endometrial cancer. Malignant tumors display genetic instability, which is classified into two types: microsatellite instability (MIN) and chromosomal instability (CIN). In the present study, we examined whether there is a relation between loss of 11q23-24 and genetic instability in endometrial adenocarcinoma. Loss of 11q23-24 was detected in 4 of 70 endometrial cancers by fluorescence in situ hybridization (FISH), and DNA aneuploidy was detected by laser scanning cytometry (LSC) in 14 tumors. All tumors with 11q23-24 loss were aneuploid, and three of them were considered to have CIN. These findings suggest that 11q23-24 contains gene(s) necessary for normal chromosome replication and cell division.