The importance of late-onset congenital adrenal hyperplasia as a cause of hirsutism is controversial. Two of 35 women with a chief complaint of hirsutism met the criteria of 21-hydroxylase deficiency. In one, who presented with hirsutism, oligomenorrhea, obesity, infertility, and enlarged cystic ovaries, the initial diagnosis was polycystic ovarian syndrome. Family data showed that her disorder was autosomal recessive and linked to the histocompatibility leukocyte antigens (HLA), as in the classic form of congenital adrenal hyperplasia. Carriers were thus detectable by HLA typing. Thus late-onset congenital adrenal hyperplasia appears to be an allelic variant of congenital virilizing adrenal hyperplasia with a milder enzymatic defect. The diagnosis cannot be made clinically because the disease has the same presentation as idiopathic hirsutism or polycystic ovarian disease. Basal plasma 17-hydroxyprogesterone levels, unlike in classic congenital adrenal hyperplasia, can be normal, and an ACTH stimulation test or sequential measurements of plasma 17-hydroxyprogesterone throughout the day may be needed to show the abnormality. The incidence among hirsute women is estimated to be 6% to 12%, and the calculated gene frequency for the allele coding for attenuated expression of 21-hydroxylase deficiency is 0.015 to 0.057.