We have developed an easy and rapid method of reverse-phase high-performance liquid chromatography (HPLC)-UV spectrometry for measuring adrenal delta 4-steroids. Three female neonates with adrenal 21-hydroxylase deficiency (2 salt-losers and 1 simple virilizer), two of whom were recalled by neonatal mass-screening for congenital adrenal hyperplasia (CAH), were diagnosed using this method. Changes of several adrenal steroids were examined in these patients before and after treatment with hydrocortisone. Before treatment, the cortisone and cortisol peaks were very low and those of 17 alpha-hydroxyprogesterone (17-OHP) and 21-deoxycortisol (21-DOF) were high in all 3 patients (17-OHP: 79.9-997 nmol/l, 21-DOF: 83.7-324 nmol/l). The androstenedione peak was also high in 2 of them. A peak produced by 21-deoxycortisone, which is a product of oxidation of 21-DOF at the C-11 position, was also detected in all cases (14.5-297 nmol/l). After treatment, all of these abnormally elevated delta 4-steroids decreased or disappeared. This new method is thought to be valuable for the rapid diagnosis of CAH, and especially for use in neonatal mass-screening for CAH.