Sixty-three women with ultrasonically detected polycystic ovaries (PCO) were investigated for a disorder of adrenal steroid biosynthesis. Serum was obtained before, and at 30 and 60 min after, the administration of 250 micrograms tetracosactrin, and assayed for 17 alpha-OH-progesterone, 21-deoxycortisol, 17 alpha-OH-pregnenolone and dehydroepiandrosterone by radioimmunoassay following paper chromatography. Results were compared with those in 11 women with normal ovaries, seven adult females with congenital adrenal hyperplasia (CAH) due to 21-hydroxylase deficiency (21OHD), and 15 women heterozygous for this defect. Although the basal-peak steroid concentration differences were significantly greater when ACTH tests were conducted between 1400 and 1700 h than between 0900 and 1000 h, absolute peak steroid concentrations were not different at either time of day. Four of 63 (6.4%) women with PCO had responses to ACTH characteristic of non-classical (late onset) 21OHD CAH, and about half the remainder had responses characteristic of 21OHD heterozygotes. There was no clear cut evidence for a deficiency in 3 beta-hydroxysteroid dehydrogenase activity in women with PCO.