The response of plasma 17-hydroxyprogesterone (17-OHP) to exogenous ACTH was investigated in 85 consecutive women referred for postmenarchal hirsutism, in order to assess the prevalence of late-onset adrenal hyperplasia due to 21-hydroxylase deficiency and its relevance to the clinical practice. An exaggerated response of plasma 17-OHP to ACTH, indicating 21-hydroxylase deficiency, was found in only one patient with a prevalence of 1.1%. The patient with late-onset adrenal hyperplasia presented signs of virilism and had high basal levels of 17-OHP. On the basis of our results, late-onset adrenal hyperplasia accounts for a very small proportion of cases of hirsutism; moreover, from this and previous studies it is apparent that the determination of basal plasma 17-OHP may be sufficient to discover late-onset 21-hydroxylase deficiency. Therefore, we do not consider ACTH testing an advisable step in the routine screening for hirsutism.