Alterations of the retinoblastoma (Rb) gene were evaluated in nine primary endometrial adenocarcinomas that we had previously analyzed for the presence of Ki-ras activations and p53 alterations and in three endometrial carcinoma cell lines. Loss of mRNA expression in the Rb gene was detected in two of the 12 tumors. Internal deletions of Rb cDNA were observed in two tumors; one was a deletion of exon 21 in a primary carcinoma, and the other was a deletion of exon 8 in one allele in one cell line. Loss of heterozygosity of the Rb gene, which was detectable by polymorphisms in introns 1 and 17, was analyzed using polymerase chain reaction-restriction fragment length polymorphism analysis in 29 endometrial carcinomas. Of 13 heterozygous cases, two cases (15%) showed loss of heterozygosity. We therefore suggest that alteration of the Rb gene, as well as activation of the Ki-ras gene and alterations of the p53 gene, plays a significant role in the etiology of endometrial adenocarcinoma.