Background: Endocrine treatments, in general have limited clinical efficacy in endometrial cancer. Upon ligand binding, PRs in normal tissue form discrete focal subnuclear distribution patterns (FDP), which are associated with DNA transcription. FDP are indicative of functionally activated PRs (APRs), and are observed in EC, independently of menopausal status. The feasibility of using an IHC technique to characterize the PR functional status has previously been reported in breast cancer and the APR phenotype in cell lines correlates with anti-progestin activity. The goal of this study is to determine if APR can be identified in EC and to determine if this IHC technique & APR phenotype could be developed as a companion diagnostic to predict anti-progestin efficacy in patients with EC. Methods: 72 archived primary EC specimens were processed with standard IHC for estrogen receptor (ER), PR & proliferation (Ki67). APR status was determined using commercially available antibodies specific to the A and B isoforms of the PR (PRA and PRB) with standard microscopy at 1000x magnification. Results: 56 (78%) tumors were of endometrioid histology. Endometrioid tumors were ER⁺ (68%) and PR⁺ (84%). Two PR nuclear distribution patterns were observed: an aggregated pattern (A) which is indicative of APR and a diffuse or finely granular pattern (D) indicative of an inactivated PR. This resulted in three tumor phenotypes: A cells only, D cells only, and a mix of A + D cells. APR was defined as any tumor with more than 5% A cells. An average of 49% of tumor cells were positive with PRA, 35% were positive for PRB. APR was present with PR A in 41% and with PR B in 47% of the PR⁺ endometrioid tumors. The APR status, for both PR A and PR B, was independent of PR positivity rate, PR staining intensity score and % Ki67 positive. APR^pos phenotype was associated with a lower % ER^posstaining. When observable, endometrial stromal and normal cells were PR positive (D pattern). Conclusions: APR can be identified using either PRA or PRB, in ~50 % of the EC samples; there was a pattern consistent with the presence of APR positive cells. The IHC technique to identify APR has the potential to be developed as companion diagnostic as a potential predictor of anti-progestin efficacy.