Mutations of the oncogene PIK3CA occur frequently in endometrial carcinomas, but their prognostic significance is unclear. To determine the clinicopathological and molecular implications of these mutations, PIK3CA status was investigated in 109 endometrial (102 endometrioid and 7 mixed) carcinomas and the results were compared with clinicopathological parameters associated with prognosis. Tumors were also investigated for microsatellite instability and PTEN, beta-catenin gene (CTNNB1), K-RAS, and B-RAF mutations. We found 35 PIK3CA somatic missense mutations in 32 (29%) endometrial carcinomas. Eighteen mutations occurred in exon 20 (kinase domain), and 17 in exon 9 (helical domain). Almost all mutated tumors were pure endometrioid adenocarcinomas. All tumors with PIK3CA mutations exhibited myometrial invasion (P=0.032). Lymphovascular invasion was found more frequently in mutated (28%) than nonmutated carcinomas (18%). Histological grade varied significantly according to the location of the PIK3CA mutations whether in exon 9 or exon 20 (P=0.033). The frequency of exon 9 mutations was higher in grade 1 carcinomas (57%) than in grade 2 (29%) or grade 3 (14%) tumors. Conversely, mutations in exon 20 were more common in grade 3 (60%) than in grade 2 (20%) or grade 1 (20%) carcinomas. None of the tumors confined to the endometrium (stage IA) had PIK3CA mutations. Furthermore, whereas 64% of adenocarcinomas with exon 9 mutations had invaded < or =(1/2) of the myometrial thickness (stage IB), 73% of tumors with exon 20 mutations had either deeper myometrial invasion (stage IC) or cervical involvement (stage II) (P=0.045). PIK3CA mutations coexisted with microsatellite instability and mutations in PTEN, CTNNB1, K-RAS, and B-RAF genes. These results favor that PIK3CA mutations are associated with myometrial invasion and, moreover, that tumors harboring PIK3CA mutations in exon 20 are frequently high-grade, deeply invasive endometrial carcinomas that tend to exhibit lymphovascular invasion.