PCOS has a hormonal pattern that includes an apparent dysfunction in ovarian steroidogenesis, although the sequence (or sequences) of endocrine changes, possibly with inherent paracrine abnormalities that can establish PCOS, are only postulated. Development of PCOS in the setting of androgen overproduction (or persistent disturbances of the hypothalamic-pituitary-adrenal axis even after normalization of androgen levels) points to adrenal enzyme defects, but the precise interaction of gonad and adrenal cortex via serum steroids remains open. Several interesting schemas for the development of PCOS have been proposed, but the subject remains controversial. Sonographic abnormalities of the ovaries may be similar in women with PCOS and women with adrenal enzyme defects, and differentiation of PCOS from nonclassical 21-hydroxylase deficiency on the basis of pituitary gonadotropin response to LHRH is also nonspecific. The 17-OHP response to ACTH testing is necessary to make the diagnosis (see nomogram). The preliminary data on the subgroup of women with PCOS and nonclassical 21-hydroxylase deficiency indicate that (1) the size of cystic changes of ovaries on sonography and (2) LH:FSH ratios may improve with dexamethasone treatment. Identification of an adrenal enzymatic disorder by ACTH testing is cost-effective in patients with symptoms of androgen excess, offers a good prognosis for improved fertility in couples, and as well is informative in family genetic studies.