The 24 hr mean plasma concentrations of estrone (E2) and estradiol (E2) were measured in 18 healthy, regularly cycling obese women; 16 healthy, regularly cycling nonobese women; 18 healthy obese men; and 33 healthy nonobese men. The obese men showed significant elevations of both E1 (67 pg/ml versus 49 pg/ml control; P less than 0.005) and E2 (37 pg/ml versus 28 pg/ml; P less than 0.005), but the obese women showed no significant elevation of either E1 or E2. The most likely explanation for the absence of significant hyperestrogenemia in the obese women despite evidence that such women have increased androstenedione-to-estrone conversion is that the latter source of estrogen is too small in comparison with estradiol secretion to cause a statistically detectable increment in plasma estrogen levels.