The steroid 17α-hydroxyprogesterone is produced by the adrenal cortex and gonads. 17α-OHP has little progestational activity, but it is of intense clinical interest because it is the immediate precursor to 11-desoxycortisol (Cpd-S). Cpd-S is produced by the 21-hydroxylation of 17α-OHP. Measurement of 17α-OHP is a useful indirect indicator of 21-hydroxlase activity. In congenital 21-hydroxylase deﬁ ciency, the most common variety of congenital adrenal hyperplasia (CAH), 17α-OHP is secreted in abundant excess. Measurement of 17α-OHP is therefore valuable in the initial diagnosis of CAH.