Although the administration of estrogens and androgens can affect the concentrations of sex hormone-binding globulin (SHBG) in men, the relationships between endogenous estrogens and androgens and SHBG are uncertain. Therefore, in a randomly selected cohort of 1640 middle-aged men we measured androgen, estrogen, and SHBG concentrations and obtained the subjects' weight, ethanol intake, and smoking histories. The data were analyzed by stepwise multiple regression, with SHBG as the dependent variable, to compare the role of hormones with that of other factors in the control of SHBG levels. Neither estrone or estradiol nor the testosterone/estradiol ratio was predictive of SHBG levels. However, SHBG concentrations were positively correlated with total testosterone and negatively correlated with percent free and percent albumin-bound testosterone. SHBG concentrations were negatively correlated with estrone sulfate, but were positively correlated with the testosterone/estrone sulfate ratio and the concentrations of free and albumin-bound testosterone. In addition, in all models tested age and body mass index (wt/ht2), but not smoking or ethanol, were strong predictors of SHBG concentrations. Thus, when present in physiological amounts in the blood as a result of glandular secretion, there is a positive relationship between SHBG concentrations and testosterone and, to a lesser extent, free- and albumin-bound testosterone, but age and body mass index appear to be more important in predicting the SHBG concentration.