An enzyme immunoassay for measuring 17 alpha-hydroxyprogesterone (17-OHP) in dried blood collected on filter paper has been developed. The method is easy and rapid and has specificity, accuracy and precision. 17-OHP values of neonates with congenital adrenal hyperplasia (CAH, 40 ng/ml) were extremely high compared with normal neonates (1.1 +/- 0.7 ng/ml). There was a negative correlation between the 17-OHP value and birth weight. The method has been applied to neonatal screening for CAH due to 21-hydroxylase deficiency. During 38 months, 67,392 neonates were screened. The recall rate and the medical evaluation rate were 1.16% and 0.09%, respectively. A third of recalled neonates were low birth weight infants. 5 neonates were proven to have CAH, and its incidence was 1:13,478. The present study demonstrates the feasibility of a neonatal screening for CAH and indicates that the frequency of CAH may be greater than previously reported by case assessment method in Japan.