To assess the role of biologically active estradiol in the development of endometrial cancer, 25 patients with endometrial tumors and a similar number of control subjects matched for age and body size were studied. No differences between the 2 groups were found for levels of total estradiol, sex hormone-binding globulin (SHBG), non-SHBG-bound estradiol, and absolute free estradiol. Body size correlated positively with levels of total, non-SHBG-bound, and absolute estradiol and negatively with SHBG levels. The obese postmenopausal women had higher total circulating levels and proportionally greater concentrations of free estradiol than nonobese subjects, suggesting a dual risk for the cellular action of circulating estradiol. These factors could contribute to the association of obesity and the occurrence of this tumor in susceptible women.