Plasma 17-hydroxyprogesterone was measured by a simple radioimmunoassay technique in six infants, aged 3 days to 3 months, with congenital adrenal hyperplasia due to 21-hydroxylase deficiency. Hormonal levels in this group were compared to those of twenty normal newborns and to those found in sixty samples of umbilical vein blood from normal deliveries. Plasma 17-hydroxyprogesterone concentrations were markedly elevated in all congenital adrenal hyperplasia infants (range 544.7-1837 nmol/l, normal 3.03-19.06) at a time when urinary studies in some of these were either not diagnostic or inconclusive. In one infant whose cord blood was analysed, the level was also greatly raised. The data suggest that early definitive diagnosis of congenital adrenal hyperplasia can be established by measurement of plasma 17-hydroxyprogesterone.