We studied the incidence of late-onset adrenal hyperplasia as a cause of hirsutism, its association with the major histocompatibility complex, and its clinical expression. Twenty-four of 400 women seen because of hirsutism were found to have late-onset adrenal hyperplasia, diagnosed on the basis of a high plasma level of 17-hydroxyprogesterone, and its marked increase after ACTH stimulation. The degree of hirsutism varied widely. Plasma antigen levels were high, especially the level of androstenedione, whereas 5 alpha-reductase activity, considered to be a good index of peripheral androgen utilization, showed frequent normal or low values. The 24 patients were genotyped, along with 84 family members, and plasma hormones were measured in the family members. We found a high correlation between late-onset adrenal hyperplasia and HLA antigens B14 and Aw33. Similar biologic profiles were observed in the patients and those of their siblings who were HLA identical (n = 9), confirming that late-onset adrenal hyperplasia is linked to the histocompatibility complex. These nine siblings had no hirsutism. We therefore conclude that the role of skin sensitivity to androgens is important in determining the clinical expression of this disorder.