Abstract Thirty-one women with idiopathic hirsutism were evaluated for partial 11- and 21-hydroxylase adrenocortical enzyme deficiencies. Twenty-four hour urine collections for 17-ketosteroids, 17-hydroxycorticoids, tetrahydro compound S (THS), and pregnanetriol were obtained basally and following a continuous 24 hour infusion of α 1–24 ACTH (cosyntropin). The results were compared to those in eight normal, nonhirsute women studied under identical conditions. Normal control subjects and 18 of 31 hirsute female patients (Group I) showed similar small increments in the excretion of THS and pregnanetriol following the infusion of cosyntropin. Thirteen hirsute women (Group II) showed cosyntropin-stimulated increments in either THS and/or pregnanetriol that were significantly greater than the mean response of the control group. The cosyntropin-stimulated increments in 17-ketosteroid excretion and basal sebum production rates were also significantly greater in Group II. The results are consistent with partial 11- and/or 21-adrenocortical hydroxylase deficiencies in some hirsute women whose condition would have previously been designated as “idiopathic”. Thus, prolonged ACTH stimulation testing may identify patients who might benefit from glucocorticoid suppression therapy.