Late-onset congenital adrenal hyperplasia is a mild genetic defect in steroidogenesis that presents with hirsutism and menstrual irregularities and responds to specific treatment with dexamethasone sodium phosphate. Its incidence as a significant cause of hirsutism or amenorrhea is controversial because it cannot be distinguished clinically from other causes. However, it is readily diagnosed by a marked increase in 17 alpha-hydroxyprogesterone levels after adrenocorticotropic hormone stimulation. Seventy-seven randomly selected women with hirsutism or amenorrhea were tested, and eight women (10.4%) were found to have late-onset congenital adrenal hyperplasia. Plasma levels of other hormones were similar in patients with and without late-onset congenital adrenal hyperplasia and were of no benefit in making the diagnosis. It is concluded that the adrenocorticotropic hormone stimulation test should be more widely utilized in patients presenting with hirsutism or menstrual dysfunction.