['To determine the effectiveness of GnRH-agonist (GnRH-a) treatment in women with late onset congenital adrenal hyperplasia.', 'Prospective assessment of GnRH-a treatment in six women with documented late-on-set congenital adrenal hyperplasia who were not preselected. Comparisons were made to previous responses in the same patients receiving dexamethasone. Eight age- and weight-matched ovulatory women served as controls.', 'Academic medical center.', 'Baseline blood determinations before and after i.v. ACTH, before and after 6 months of GnRH-a treatment. Estrogen and progestin replacement was begun in all women after the 3rd month of treatment.', 'Serum 17-hydroxyprogesterone (17-OHP), gonadotropin, and androgen levels before and after GnRH-a treatment. Responses of 17-OHP and androgens to ACTH assessment of hirsutism using a modified Ferriman-Gallwey score.', 'Gonadotropins, estrogen, androgen, and 17-OHP were suppressed with GnRH-a treatment. Levels were similar before and after estrogen and progestin replacement. Responses of 17-OHP after ACTH were blunted but still were elevated compared with responses in controls. Ferriman-Gallwey scores decreased significantly (-8 +/- 1; mean +/- SE). This response was greater than that observed previously with 6 months of dexamethasone (-2 +/- 0.3).'] Does ovarian suppression reduce clinical and endocrine expression of late-onset congenital adrenal hyperplasia due to 21-hydroxylase deficiency?