This study was undertaken to examine the role of adrenal androgen excess in the pathogenesis of polycystic ovary syndrome (PCOS) and, if such was present, to assess its reversibility using dexamethasone given in physiological dosage at night. Mean plasma testosterone (T), T/sex-hormone binding globulin (T/SHBG) ratio, androstenedione, and 17-OH-progesterone levels were elevated in the 19 patients studied. Plasma estrone values were elevated, whereas estradio levels were normal. Plasma FSH was decreased and LH responsiveness to LHRH was exaggerated. Metyrapone, an 11-hydroxylase inhibitor, was administered at 2400 h induce hypocortisolemia and compensatory ACTH secretion so that adrenal androgen and glucocorticoid responsiveness to endogenous stimulation could be examined. Plasma T, androstenedione, and 11-deoxycortisol responses to metyrapone were excessive in PCOS patients, thus indicating a specific adrenal abnormality. After 3 months treatment with dexamethasone, 0.5 mg at night, mean plasma T/SHBG and androstenedione declined to normal, and mean plasma dehydroepiandrosterone and dehydroepiandrosterone sulfate declined to below normal. The mean estrone value was slightly lower during dexamethasone. Plasma LH responsiveness to LHRH was no longer significantly different from normal, but FSH was suppressed. During treatment androgen responsiveness to metyrapone stimulation was normal, whereas 11-deoxycortisol responsiveness was suppressed. Fifteen patients completed 3 months of treatment with dexamethasone. Of these, 10 resumed regular menstruation. The latter group had suppression of plasma T, T/SHBG, androstenedione, dehydroepiandrosterone, and dehydroepiandrosterone sulfate. Only plasma androstenedione fell significantly in the remainder. These observations support the hypothesis that, in at least some patients, PCOS develops in response to abnormal gonadotropin secretion induced by hyperestronemia occurring as a consequence of excessive adrenal androgen secretion.