Thirty-one cases of idiopathic hirsutism, characterized biochemically in the basal state by increased levels of urinary 3 alpha-androstane-5 alpha, 17 beta-diol and normal levels of the main androgens, were studied. In order to determine a possible etiologic heterogeneity of idiopathic hirsutism, pituitary gonadotropin responses to synthetic luteinizing-releasing hormone (LRH) and adrenal steroid responses to adrenocorticotropic hormone (ACTH) stimulation were evaluated and the results were compared to those in six normal women. On the basis of the results obtained in each hirsute patient after LRH and ACTH tests, two groups were identified. The majority, 23 of 31 hirsute patients (group I), had results similar to those in the control group. In the other eight patients (group II), biologic abnormalities were disclosed and suggested a partial adrenal 11 beta-hydroxylase defect in two patients, an incomplete form of adrenal 3 beta-ol deficiency in one patient, an adrenal hyperreactivity without evident cause in two patients, and polycystic ovary syndrome in association with an adrenal hyperreactivity in three patients. As a group, the eight patients showed ACTH-stimulated increments in testosterone, delta 4-androstenedione, dehydroepiandrosterone, and 17-ketosteroids that were significantly greater (p less than 0.01) than the mean responses in the control group. The conclusion is that some women who previously were designated as having "idiopathic" hirsutism had an adrenal and/or ovarian component to their hyperandrogenism which could be shown only by appropriate dynamic tests.