Objective. Endometrial carcinomas are divided into type I endometrioid endometrial carcinomas (EECs), thought to arise from hyperplastic endometrium, and type II nonendometrioid endometrial carcinomas, thought to arise from atrophic endometrium. However, a minority (20%) of EECs have atrophic background endometrium, which was shown to be a marker of a worse prognosis. This study compares the immunohistochemical and genetic profiles of this possible third type to that of the known two types. Methods. 43 patients with grade 1 EEC and hyperplastic background endometrium (type I), 43 patients with grade 1 EEC and atrophic background endometrium (type III) and 21 patients with serous carcinoma (type II) were included (n = 107). Tissue microarrays of tumor samples were immunohistochemically stained for PTEN, L1CAM, ER, PR, p53, MLH1, PMS2, beta-catenin, E-cadherin and MIB1. The BRAF, KRAS, and PIK3CA genes were analyzed for mutations. Results. A significantly higher expression of ER and PR, and a lower expression of L1CAM, p53 and MLH1 were found in type I and III compared to type II carcinomas. Expression of E-cadherin was significantly reduced in type III compared to type I carcinomas. Mutation analysis showed significantly less mutations of KRAS in type III compared to type I and II carcinomas (p < 0.01). Conclusion. There appear to be slight immunohistochemical and genetic differences between EECs with hyperplastic and atrophic background endometrium. Carcinogenesis of EEC in atrophic endometrium seems to be characterized by loss of E-cadherin and a lack of KRAS mutations. As expected, endometrioid and serous carcinomas were immunohistochemically different. (C) 2015 Elsevier Inc. All rights reserved.