We have adapted a RIA to the measurement of 17α-hydroxyprogesterone (17α-hydroxypregn-4-ene-3, 20-dione (17-OHP)] in urine and have evaluated it as a tool in diagnosis and management of congenital adrenal hyperplasia (CAH). In 51 normal subjects, aged 3 days to 57 yr, the mean excretion of 17-OHP was 200 ng/day, with a range of 21-799 ng/day. As a ratio of 17-OHP to creatinine excreted in 7 normal infants, aged 3 days to 1 month, the mean was 4.13 ng/mg creatinine, with a range of 0.77-9.70 ng/mg creatinine; the mean for the remaining 44 subjects, aged 2-57 yr, was 0.21 ng/mg creatinine, with a range of 0.04-0.47 ng/mg creatinine. In urine samples from 13 children hospitalized with nonadrenal illnesses, the 17-OHP excretions, either as total per 24 h or as the ratio of 17-OHP to creatinine excreted, were similar to values seen in normal children of comparable age. In 6 new cases of CAH diagnosed in infancy, the urine 17-OHP ranged from 544-4370 ng/day (18.0- 140 ng 17-OHP/mg creatinine), whereas the urine pregnanetriol (5β-pregnane-3α, 17α, 20α -triol) in 1 of the neonates was unmeasurable and in 2 others was only equivocally elevated. There was good correlation (r = 0.853) between 17-OHP and pregnanetriol in 24-h urine collections obtained from 34 patients during therapy of CAH. As a biochemical parameter in monitoring therapy of CAH, 17-OHP in urine may prove superior to pregnanetriol, particularly in neonates who may not excrete elevated levels of pregnanetriol.