Plasma 17-hydroxyprogesterone (17-OHP) was estimated in 9 infants aged 6 to 12 days with congenital adrenal hyperplasia (CAH) due to 21-hydroxylase deficiency. Raised plasma 17-OHP values, ranging from 3·4 to 25·0 μg/100 ml were found. 50 infants aged 10 hours to 15 days with normal adrenal function were also studied. One infant with galactosaemia had 17-OHP levels of 1·8 and 2·4 μg/100 ml, and a further 3 infants aged 15 hours to 3 days had 17-OHP levels between 1·1 and 2·1 μg/100 ml. In the remaining infants, plasma 17-OHP was less than 1·0 μg/100 ml. It therefore appears that estimation of plasma 17-OHP provides a useful method for confirming the diagnosis of CAH during the newborn period.