Clinical outcome of 23 patients with mixed endometrioid and serous endometrial carcinomas (mixed EEC-SC) was compared to that of pure endometrioid (EEC) and pure serous (SC) carcinomas. Hotspot mutation frequencies in KRAS , PIK3CA , PTEN , and TP53 and microsatellite instability (MSI) status were determined in mixed EEC-SC, as well as in their EEC and SC microdissected components separately, and alterations were compared to frequencies in pure EEC and SC. Relapse-free (RFS) and overall survival (OS) differed significantly between mixed EEC-SC and pure EEC and SC, revealing that outcome of mixed EEC-SCs was intermediate to that of pure EEC and pure SC. PTEN mutations were absent in pure SC, but occurred in 20 % of pure EEC, and 13 % of mixed EEC-SC. In contrast, TP53 mutations were more frequent in pure SC (17 %) and mixed EEC-SC (22 %) than in pure EEC (2 %). Mutations in mixed EEC-SC were shared by the two microdissected components in 30 %, whereas in 35 %, some mutations were component-specific. Mutation analysis confirms similarities between the EEC and SC components of mixed EEC-SC with pure EEC and pure SC, respectively. However, PTEN and KRAS mutations were more frequent in the SC component of mixed EEC-SC than in pure SC, while TP53 mutations were more frequent in the EEC component of mixed EEC-SC than in pure EEC. Presence of different clonal mutation pattern between EEC and SC components of mixed EEC-SC raises the possibility of divergent tumor heterogeneity or biclonal origin in some cases.