Objective: To determine the frequency of late-onset adrenal hyperplasia (LOCAH) due to 21-hydroxylase (21-OH) and 11b-hydroxylase (11b-OH) deficiency in women with clinical and biochemical features of polycystic ovary syndrome (PCOS).Design: Eighty-three consecutively selected women with PCOS and eighteen normal women were included in the study.Methods: Ultrasound, clinical and hormonal parameters were used to define PCOS. Basal FSH, LH, testosterone, free testosterone, androstenedione, dehydroepiandrosterone sulfate (DHEA-S), sex hormonebinding globulin (SHBG) and cortisol levels were measured. Serum 17-hydroxyprogesterone (17-OHP) and 11-deoxycortisol (11-DOC) levels were also measured before, 30 and 60 min after a single bolus injection of 0.25 mg ACTH (1-24) at 0900 h during the mid-follicular phase of the cycle. ACTHstimulated 17-OHP levels >30 nmol/l were considered as the criteria of 21-OH deficiency. The diagnosis of 11b-OH deficiency was made if the adrenal 11-DOC response to ACTH stimulation exceeded threefold the 95th percentile of controls.Results: Basal serum testosterone, free testosterone, androstenedione, DHEA-S, cortisol and 11-DOC levels were significantly higher in PCOS than in control subjects. ACTH-stimulated 17-OHP (P < 0:05) and 11-DOC (P < 0:0005) levels were found to be significantly higher in patients with PCOS than in controls. Seven (8.4%) patients had an 11-DOC response to ACTH higher than threefold the 95th percentile of controls, while no patients showed evidence of 21-OH deficiency.Conclusions: We have found that 8.4% of the women with clinical and biochemical features of PCOS could be presumed to have 11b-OH deficiency. No patients among the women with PCOS showed evidence of 21-OH deficiency. 11b-OH deficiency is unexpectedly more common than 21-OH deficiency in women with PCOS.