It is known that plasma total testosterone (T) is decreased in obese men in proportion to the degree of obesity, but similar information is not available for plasma free T and non-sex-hormone-binding globulin (SHBG)-bound T. We measured the 24-h mean plasma total T in 48 healthy (non-weight-stable men, aged 18-55 yr, with body mass indexes (BMI) ranging from 21-95 kg/m2. Free T and non-SHBG-bound T were calculated using the measured total T, the concentrations of albumin and SHBG, and the association constants of T to albumin and SHBG. Total body fat content was measured by deuterium-water isotope dilution. Findings were as follows. 1) BMI was very highly correlated with total body fat content (r = 0.96; P less than 0.001); thus, the degree of obesity can be calculated just as appropriately from simple height and weight measurements as from measurements of total body fat content. 2) Total, non-SHBG-bound, and free T were all highly correlated inversely with BMI; for total T, r = -0.727, P less than 0.01; for non-SHBG-bound T, r = 0.677, P less than 0.01; and for free T, r = -0.653, P less than 0.01. Thus, free T and non-SHBG-bound T are decreased in obese men in proportion to the degree of obesity, just as is the case for total T; percentage-wise, the decrease was the same for all 3 parameters.