Recognition of high-grade endometrial stromal sarcoma is important because of its aggressive clinical behavior. Morphologic features of YWHAE-NUTM2 high-grade endometrial stromal sarcoma may overlap with other uterine sarcoma types. BCOR immunoexpression was studied in these tumors and their morphologic mimics to assess its diagnostic utility. BCOR immunohistochemical staining was performed on archival tissue from 28 high-grade endometrial stromal sarcomas with classic morphology (20 YWHAE-NUTM2, 5 ZC3H7B-BCOR, 3 BCOR-ZC3H7B), 3 high-grade endometrial stromal sarcomas with unusual morphology and unknown gene rearrangement status, 66 low-grade endometrial stromal sarcomas, 21 endometrial stromal nodules, 38 uterine leiomyosarcomas, and 19 uterine leiomyomas. Intensity of nuclear staining and percentage of positive tumor cells were recorded. Strong diffuse nuclear BCOR staining (defined as >95% of tumor cells) was seen in the round cell component of all 20 (100%) classic YWHAE-NUTM2 high-grade endometrial stromal sarcomas and the 3 unusual high-grade endometrial stromal sarcomas which prompted FISH studies confirming YWHAE rearrangement in 2 tumors. Genomic PCR confirmed the presence of BCOR exon 16 internal tandem duplication in the third case. Diffuse BCOR staining was strong in three and weak in one BCOR-rearranged high-grade endometrial stromal sarcoma while absent in the remaining four BCOR-rearranged tumors. BCOR staining was weakly positive in <5% of tumor cells in 4 of 66 (6%) low-grade endometrial stromal sarcomas and 1 of 18 (6%) endometrial stromal nodules and weakly to moderately positive in <5-40% of tumor cells in 6 of 31 (19%) leiomyosarcomas. No BCOR staining was seen in the remaining low-grade endometrial stromal sarcomas, endometrial stromal nodules, leiomyosarcomas, or any of the leiomyomas. BCOR immunohistochemical staining is a highly sensitive marker for YWHAE-NUTM2 high-grade endometrial stromal sarcoma with both classic and unusual morphology and identifies a subset of high-grade endometrial stromal sarcoma with BCOR alterations, including BCOR rearrangement and internal tandem duplication.