It has been shown by Strottet al.in 1969 that 17 α‐hydroxy progesterone (17‐OHP) was the main plasma steroid in patients with virilizing congenital adrenal hyperplasia (CAH) due to a deficiency in 21 hydroxylase. Estimation of this steroid in the plasma of children however was not readily performed until the adaptation of the technique of competitive protein binding (CPB). Barnes and Atherden (1972) showed that by using such a CPB assay, the determination of serum 17‐OHP was of value in the diagnosis of CAH due to 21 hydroxylase deficiency. This paper reports the experience gained at the Royal Children's Hospital Melbourne using a CPB assay for 17‐OHP in the diagnosis of patients with suspe