Relationships between plasma sex hormones and different parameters of obesity (weight, ideal body weight [IBW], overweight, fat mass, and body surface) were investigated in 70 healthy nonobese and obese males, 20-40 yr of age and with a body weight of 85%-245% of IBW. Plasma sex hormones remained unaffected by weight up to approximately 160% of the IBW. Only in the massively obese subjects was plasma testosterone decreased to 40% of controls (from 6.2 to 2.5 ng/ml), whereas free testosterone remained almost constant. On the other hand, plasma estrone and estradiol exhibited significant increases in obese subjects, ranging from 31.5 +/- 52.3 +/- 5.8 pg/ml for estrone, and 25.4 +/- 5.4 increasing to 44.7 +/0 5.0 pg/ml for estradiol. Similarly, free estradiol was shown to significantly increase with obesity in men from 505 +/- 118 to 991 +/- 123 fg/ml (p < 0.001). The ratios of testosterone/androstenedione, as well as of estradiol/estrone, were not affected by obesity, suggesting that reduction of the 17-oxo-group of the steroids is not influenced by the amount of fat tissue. A significant (p < 0.001) correlation was found between IBW and estrone (r = 0.80) and estradiol (r = 0.75), as well as the ratios of estrone/androstenedione (r = 0.62) and estradiol/testosterone (r = 0.86). This is consistent in its evidence indicating that fat tissue may be able to aromatize androgens. In the obese subjects, there were significant correlations between plasma sex hormones (testosterone, estrone, estradiol, and free estradiol) and the parameters of obesity used. Among these, correlations were best with IBW, overweight, and fat mass (r = 0.74-0.89; p < 0.001); body weight and body surface were less favorable.