The histological and ultrastructural features of 15 endometrial carcinomas were analyzed and recorded without knowledge of the steroid receptor content which was determined in the cytosol of the tumor tissue. Serum levels of estrogens were determined in all patients. The importance of this simultaneous determination of the levels of circulating steroid hormones is emphasized. Some degree of correlation was found between the histological grading of the neoplasms and their steroid receptor content. On the contrary, no evident relation seems to exist between steroid hormone-binding capacity and ultrastructural features of the tumor cells. These different results are presented and discussed.