A series of 183 cases of primary endometrial carcinomas were immunohistochemically stained for estrogen receptors (ERs) and progesterone receptors (PRs) using formalin-fixed, paraffin-embedded sections. All specimens were obtained from uterine curettages performed at the time of the initial diagnosis or initial therapy. The ER and PR content in the malignant and benign components (benign epithelium, stroma, and myometrium) was evaluated separately for positivity and divided into three groups according to the percentage of positive cells and the intensity of the nuclear stain. Endometrioid-type adenocarcinoma had the highest degree of positivity for both receptors, followed by adenosquamous carcinoma, serous carcinoma, and clear cell carcinoma. The positivity for ERs and PRs of the malignant component was statistically correlated with the International Federation of Gynecology and Obstetrics (FIGO) stage (P less than 0.01), FIGO grade (P less than 0.001), and nuclear grade (P less than 0.0001) of the tumors. The degrees of ER and PR positivity of the malignant component correlated with each other (P less than 0.0001). There was no association between the depth of myometrial tumor invasion and either receptor status of the malignant component. There was a significant association between the presence of lymph vessel invasion and the positivity for PR. Positivity for PR correlated negatively with the patients' ages (P less than 0.004). The results of this study indicate that immunohistochemical analysis of sex steroid receptor status on formalin-fixed, paraffin-embedded tissue offers an excellent alternative to the standard biochemical procedure.