A previous study evaluated two types of endometrial carcinoma: one with, the other without, associated adenomatous hyperplasia (Groups 1 and 2, respectively). On the basis of histologic observations, the study concluded that Group 1 tumors were likely to be estrogen-dependent, whereas Group 2 tumors appeared to represent a hormone-independent type of cancer. The authors present quantitative biochemical data to support the proposition that Group 1 tumors are likely to be under the influence of estrogen. Progesterone receptor levels, known to be increased by estradiol, were significantly higher in Group 1 than in Group 2. Analysis of histologic variants revealed that some histologic components, such as mucinous adenocarcinoma, squamous components, and stromal foam cells, predominated in Group 1 and may, therefore, be indicative of estrogen action. Certain histologic features, however, such as papillary carcinoma, clear cells, and anaplastic carcinoma with giant tumor cells, were found exclusively in Group 2, thus suggesting a lack of estrogenic influence. These findings indicate that Group 1 and Group 2 endometrial carcinomas may constitute two different biologic entities and, thus, encourage further basic and clinical research to develop specific therapies for patients diagnosed to have endometrial cancer.