It has been postulated that hirsute patients may have a relative deficiency in 11.beta.-hydroxylase activity of the adrenal cortex. To test this postulate, serum levels of cortisol (Cp F) and 11-desoxycortisol (Cp S) were measured, and the Cp S/Cp F ratio in 9 nonhirsute and 34 hirsute premenopausal women was estimated. As a group, the hirsute patients had significantly elevated (P < 0.05) mean Cp F and S levels, but the mean Cp S/Cp F ratio was not significantly different from normal. Considered individually, only 3 hirsute patients had a Cp S/Cp F ratio greater than 2 SD above the mean normal levels. These ratios were 0.0218, 0.0139 and 0.023. If there is indeed an 11.beta.-hydroxylase deficiency in these 3 patients, it must be relatively minor, since a patient with documented 11.beta.-hydroxylase deficiency had Cp S levels of 218 ng/ml and a Cp S/Cp F ratio of 0.7. 11.beta.-Hydroxylase deficiency is probably not a common cause of hirsutism.