Serum 17 beta-estradiol (E2) and estrone (E1) levels were measured before and 6-8 weeks after ovariectomy in 16 postmenopausal women with endometrial cancer, and in 10 postmenopausal women with normal endometrium (preovariectomy only). For E2, no significant difference in the mean baseline (+/-SE) level was found between the tumor (13.5 +/- 1.1 pg/ml) and non-tumor patients (11.7 +/- 1.4 pg/ml). For the same patients, the mean baseline E1 levels of 33.8 +/- 2.4 pg/ml and 28.5 +/- 4.7 pg/ml were also not significantly different. The mean body weight of the tumor patients (141.1 +/- 7.3 lbs.) was similar to the mean weight of the non-tumor subjects (137.5 +/- 6.3 lbs.). This was of importance since both E2 and E1 levels correlated significantly with body weight and excess fat in these postmenopausal women. The circulating estrogen levels did not correlate significantly with the patients' height, age, or years of menopause. In the patients with endometrial cancer the mean E2 (14.1 +/- 1.7 pg/ml) and E1 (39.5 +/- 7.3 pg/ml) levels after ovariectomy were not significantly different from the preoperative concentrations. These data are consistent with the concept that in postmenopausal women most, if not all, circulating estrogen is produced by peripheral conversion of androgens and that this conversion is influenced by obesity. Circulating estrogen levels are not significantly different in postmenopausal patients with endometrial cancer compared with women of a similar age and weight who do not have the tumor.