Hereditary nonpolyposis colorectal cancer (HNPCC) is characterized by a familial predisposition to colorectal carcinoma and extracolonic cancers of the gastrointestinal, urological, and female reproductive tracts, notably the endometrium. A genetic locus for HPNCC was recently determined by linkage analysis to exist on chromosome 2p; both sporadic and HNPCC-associated colorectal carcinomas exhibit a "replication error" phenotype, characterized by instability of dinucleotide repeat sequences throughout the genome. Here, we address the hypothesis that the replication error phenotype would be evident in some fraction of sporadic endometrial carcinomas or in those associated with HNPCC. Microsatellite instability was observed in 17% of sporadic endometrial carcinomas and in 75% of those tumors associated with HNPCC. These data indicate that the HNPCC gene is also involved in heritable and somatic forms of endometrial carcinoma.