Objective: We sought to evaluate the expression of G protein-coupled receptor 30 (GPR30) and estrogen receptor (ER)β in uterine carcinosarcoma (CS). Study Design: Immunohistochemistry was performed using antibodies to GPR30, ERβ, ERα, and progesterone receptor (PR). The staining intensity and percentage of positive cells were scored for each tissue section. Expression levels were compared using the Wilcoxon rank sum test. Correlation was evaluated by Spearman rho and logistic regression. Results: Compared with normal endometrium, CS had lower ERα and PR expression (both P < .01) but higher GPR30 epithelial expression (P = .03). Advanced-stage CS had higher GPR30 (P < .01) and ERβ (P = .02) epithelial expression compared with early-stage CS. Expression of GPR30 and ERβ correlated with each other (P < .01), and not with ERα or PR. Conclusion: In uterine CS, GPR30 and ERβ are coordinately overexpressed and expression levels increase in advanced-stage disease, supporting the involvement of alternative ERs in disease progression. © 2010 Mosby, Inc. All rights reserved.