Attenuated or partial 21-hydroxylase deficiency is one of several biochemical defects in steroid metabolism that can lead to hirsutism in young women after puberty. The diagnosis is made through the exaggerated response of 17 alpha-hydroxyprogesterone (17-OH) to adrenocorticotrophic hormone (ACTH). To provide reference data 72 mild to moderately hirsute patients aged 18 to 35 were studied (over two years) with the ACTH test. Four patients with an exaggerated response were found. The mean (+/- 1SD) for zero time was 2.4 (0.96) nmol/l and for the 60 minute time was 7.2 (2.04) nmol/l. A subpopulation was found with a significantly higher baseline at 6.2 (1.3) nmol/l but a blunted 60 minute response at 8.7 (2.5) nmol/l. This is important because of the potential confusion arising from the known variability in baseline values in previously reported patients with partial 21-hydroxylase deficiency. Extending the test to 90 minutes did not show further increase in the 17-OH response to ACTH, thus confirming the validity of the 60 minutes ACTH test. The cortisol response to ACTH was also studied. One patient with presumptive partial 21-hydroxylase deficiency overlapped in cortisol response with eight of the reference population. Theoretically, a blunted cortisol response would be expected because of the postulated enzyme block, and these results suggest that other steroid enzyme defects should also be considered when an exaggerated 17-OH response to ACTH is seen.