Plasma estrogen levels (estrone, estradiol and estriol) in post-menopausal subjects with documented endometrial carcinoma were measured by sensitive and specific radio-immunoassay techniques and were compared with suitable control subjects. No estriol levels significantly different from zero were obtained in either the control or affected populations. Patients with endometrial carcinoma showed significantly higher mean estrone (E1) and estradiol (E2) levels than those of the controls. No significant differences were found, within each grouping, for both E1 and E2 as a function of age, time from menopause, or diabetic status as determined by 3 hour oral glucose tolerance test. For both E1 and E2, a positive correlation with obesity was found for patients with endometrial carcinoma, but no correlation was observed for control subjects for either estrogen. These studies provide data which substantiate the previously suspected link between elevated estrogen levels and endometrial carcinoma. Work is continuing to further define the hormonal milieu and to extend these investigations to cancer of the breast.