We measured serum sex hormone-binding globulin (SHBG), total testosterone (T), non-SHBG-bound T, albumin-bound T, free T, and SHBG-bound T in 19 prepubertal boys with hypopituitarism. Serum SHBG decreased with age with a slope similar to that in 91 normal prepubertal boys at higher level, and therefore, it reached similar values at a later age. Serum SHBG was significantly higher in hypopituitary prepubertal boys [mean, 123 +/- 12 (+/- SE) nmol/L] than in normal prepubertal boys (76 +/- 4; P less than 0.001) despite the fact that their mean age was also higher (10.0 +/- 4 vs. 7.1 +/- 4.1 yr; P less than 0.001). In 4 boys with isolated hypogonadotropic hypogonadism (Kallman's syndrome), aged 15.6 +/- 1.5 yr, serum SHBG was 21 +/- 14 nmol/L, a value below the 95% confidence limit of the regression line in GH-deficient boys. The affinity constants of association of the SHBG-DHT complex were similar in hypopituitary and normal boys. Eleven of the 19 hypopituitary boys (mean chronological age, 8.3 +/- 2.5 yr; mean bone age, 4.1 +/- 2.1 yr) were treated with recombinant hGH (0.5 U/kg BW.week) for 1 yr. Their mean serum SHBG level before treatment was 154 +/- 14 nmol/L, and it decreased gradually to 106 +/- 5 nmol/L (P less than 0.01) after 12 months of treatment. The tendency toward normalization of serum SHBG during treatment suggested that GH deficiency was responsible for the high serum SHBG levels. Serum SHBG correlated negatively with age in both treated hypopituitary and normal boys, but the slope of the regression line was significantly steeper in treated hypopituitary boys (P less than 0.01). On the other hand, the mean serum non-SHBG-bound T level was 0.10 +/- 0.02 (+/- SE) nmol/L in hypopituitary boys, significantly lower than that in normal boys (0.21 +/- 0.02 nmol/L; P less than 0.02). Since serum total T concentrations were similar in the two groups, the higher serum SHBG concentration resulted in lower serum bioavailable T levels in the hypopituitary boys. These changes might explain the poor response to T treatment reported in GH-deficient patients. The lower serum non-SHBG-bound T concentrations in the GH-deficient boys suggest there may be delayed exposure of central nervous system structures to increased levels of sex hormones, which, in turn, may slow body maturation. This mechanism might play a role in the delay of puberty that occurs in patients with isolated GH deficiency.