In endometrial carcinomas (ECs), previous report suggested that PIK3CA mutations do not coexist with KRAS mutations, but the significant mutual exclusiveness has not been demonstrated. In this study, we examined the mutation frequency of PIK3CA in EC and its mutual exclusiveness with KRAS mutation. We performed mutational analysis of PIK3CA through a polymerase chain reaction single-strand conformation polymorphism assay in 44 cases of endometrial cancer and analyzed the correlation with loss of PTEN, KRAS mutation, and RASSF1A hypermethylation. Somatic mutations of PIK3CA were detected in 14 of 44 (31.8%) of endometrial cancers. In exon 9, seven PIK3CA mutations were located, while seven mutations were located in exon 20. The most common mutation was E545A (35.7%), followed by H1047R (28.6%). Concomitant loss of PTEN expression and PIK3CA mutation was found in four cases of endometrial cancer. KRAS mutations were mutually exclusive with PIK3CA mutations, and those mutations were inversely correlated with statistical significance (P = 0.039). Also, we found that mutations in ERBB2 were mutually exclusive with PIK3CA mutations. RASSF1A and hMLH1 methylation were not correlated with the presence of PIK3CA mutations. PIK3CA was frequently mutated in endometrial cancers. KRAS and PIK3CA mutations are inversely correlated, suggesting that genetic alterations of KRAS and PIK3CA may play equivalent roles in endometrial carcinogenesis.