The purpose of this study was to ascertain if a relationship exists between the transfer constant of conversion of plasma androstenedione to estrone ([rho]AE1BU) and total body weight or excessive body weight in 50 postmenopausal women, of whom 25 had adenocarcinoma of the endometrium and 25 had no endometrial disease. The [rho]AE1BU ranged from 0.015 to 0.129 in these 50 women. The [rho]AE1BU in the women with endometrial cancer was 0.051 +/- 0.006 (mean +/- S.E.), whereas that in the women with no endometrial disease was 0.039 +/- 0.004. These values are not significantly different (p greater than 0.05). The body weights of these 50 women ranged from 104 to 430 pounds. The weight of the patients with endometrial cancer was 234 +/- 16 pounds (mean +/- S.E.), and that for the women with no endometrial disease was 194 +/- 12 pounds. A statistically significant correlation (p less than 0.001) was found between [rho]AE1BU and body weight and between [rho]AE1BU and excessive body weight in both groups of women. Moreover, obesity and aging appear to act in concert to potentiate the conversion of plasma androstenedione to estrone in extraglandular sites since the [rho]AE1BU is considerably greater among obese postmenopausal women than among comparably obese premenopausal women.