Both the androgen-induced decline in serum sex hormone-binding globulin (SHBG) levels during puberty and the anabolic effect of exogenous testosterone are absent in patients with androgen insensitivity (testicular feminization). To determine whether the androgen-induced decline in serum SHBG could be used as a test of androgen sensitivity, we studied the effect of the anabolic-androgenic steroid stanozolol (17 beta-hydroxy-17 alpha-methyl-5 alpha-androstano-[3,2-c]pyrazol) on serum SHBG in 25 control subjects, 3 patients with complete androgen insensitivity, and 4 patients with partial androgen insensitivity. Stanozolol was administered orally for 3 days (0.2 mg/kg.day); blood samples were taken before and 5, 6, 7, and 8 days after the beginning of the test for measurements of serum SHBG. The lowest value (i.e. the peak response) in each subject was used as the measure of the response to stanozolol. In the control subjects the mean nadir serum SHBG level was 51.6 +/- 5.9% (+/- SD) of the initial value (P less than 0.001). In the 4 patients with partial androgen insensitivity the nadir serum SHBG ranged from 73-89%, and in the 3 patients with complete androgen insensitivity it ranged from 93-97% of the initial value. Thus, the decrease in serum SHBG after short term administration of stanozolol reflects androgen responsiveness and, thus, may be used to differentiate patients with androgen insensitivity syndromes from those with other causes of male pseudohermaphroditism.