Progesterone inhibits the proliferative growth effects of estrogen in the endometrium and prevents the development of endometrial hyperplasia and Type I adenocarcinoma. The exact mechanism of this action is unknown. The progesterone-induced helix-loop-helix transcription factor Heart and Neural Crest Derivatives Expressed 2 (Hand2) was recently shown to suppress production of growth factors in the endometrium. In Hand2 knockout mice, continuous proliferation of the endometrium was observed. In this study, archival paraffin-embedded tissue from 56 hysterectomy specimens was examined by immunohistochemistry for the expression and localization of Hand2, estrogen receptor (ER), progesterone receptor (PR), and Ki-67. Diagnoses included disordered proliferative endometrium, simple and complex hyperplasia with or without atypia, and endometrioid adenocarcinoma. Hand2 expression is localized to endometrial stromal nuclei. In benign endometrium, Hand2 expression was moderate to strong (10/11; 91%), with weak Hand2 expression in only 1 case (1/11; 9%). Similar Hand2 expression patterns were observed in disordered proliferative endometrium and simple hyperplasia without atypia, with moderate to strong expression in 91% of cases (10/11) and weak expression in 9% of cases (1/11). In contrast, simple and complex hyperplasia with atypia exhibited moderate to strong Hand2 expression in 8% of cases (1/12) and a loss of expression or weak expression in 92% of cases (11/12). In endometrioid adenocarcinomas, Hand2 expression was absent in all cases (22/22). Hand2 is expressed in the stroma of benign endometrium, but expression is significantly reduced or lost in atypical hyperplasia and endometrioid carcinoma. Thus, the absence of Hand2 expression may be a useful biomarker for atypical hyperplasia and endometrioid carcinoma.