The 24 h mean plasma concentrations of estrone sulfate (ES) were measured in 27 healthy obese and nonobese men (BMI: 21.2-89.5). Plasma levels of ES were found to be elevated in obese men, with ES values significantly correlated to the level of obesity (r = 0.60; P less than 0.001). Thus, an increase in plasma ES concentration (from 524 to 1115 pg/ml), compared to the less than 40 percent increases previously found for estrone and estradiol. Because ES is normally present at an approximately tenfold greater concentration than either estrone or estradiol in men, it may serve as a more easily measurable indicator of adipose tissue aromatization of androstenedione.