In a longitudinal study of 82 children we found a gradual rise in median plasma concentrations of 11 beta-hydroxyandrostenedione (11 beta-OH-A4) from 2.5 to 6.4 nmol/l during childhood which was similar in both sexes. This could reflect changes in adrenal function during the adrenarche and sexual maturation. Plasma concentrations of 11 beta-OH-A4 in adults follow the patterns of cortisol secretion. In patients with diseases of the adrenal cortex, the plasma concentrations of 11 beta-OH-A4 were consistent with the pathology of each condition. In women with polycystic ovaries (PCO) undergoing gonadotrophic stimulation for in vitro fertilization and embryo transfer, 11 beta-OH-A4 (median = 3.8 nmol/l), testosterone and androstenedione, were raised when compared to women with normal ovaries (11 beta-OH-A4 median = 2.6 nmol/l). Follicular fluid has concentrations of 11 beta-OH-A4 six to twelve times greater than plasma levels and in women with PCO, 11 beta-OH-A4 concentrations were lower than in women with normal ovaries, which is consistent with an inhibition of ovarian 11 beta-hydroxylase. Granulosa cells in vitro demonstrated the production of 11 beta-OH-A4 by side chain cleavage of cortisol. These data support an adrenal source for 11 beta-OH-A4 but the raised plasma concentrations in women with polycystic ovary syndrome (PCOS) may reflect the excess androgen output from the ovary. 11 beta-OH-A4 may therefore be an additional marker for ovarian dysfunction.