Fifty-two postmenopausal women with newly diagnosed endometrial carcinoma and 58 postmenopausal age-matched controls were studied concerning serum levels of estrone, total estrone, estradiol, androstenedione, testosterone, dehydroepiandrosterone and its sulphate, sex steroid hormone binding globulin, follicle stimulating hormone, and luteinizing hormone. The patients had a higher mean serum level of estradiol (P = 0.006) and a lower level of follicle stimulating hormone (P = 0.001) than the controls and the significant differences remained after the number of years since the menopause and body index had been taken into account. As the serum levels of steroid hormone binding globulin tended to be lower among patients than among controls (P = 0.084), the difference in the biological effect of estradiol between the two groups was probably greater than the difference in serum concentrations would indicate. No significant difference in the ratio of estrone to androstenedione or in the mean serum level of androstenedione was found between patients and controls. These data support the role of estrogen in the etiology of endometrial carcinoma.