During recent years the metabolic disorder in congenital adrenal hyperplasia that leads to pseudohermaphroditism in females has been a subject of great interest. This was stimulated by the dramatic demonstration by Wilkins et al. (1950) that administration of cortisone led to a remarkable relief of the symptoms and apparently restored the disordered metabolism to normal. Many diseases of uncertain aetiology are relieved by cortisone even when adrenal steroid metabolism appears to be normal, so that no conclusions could be drawn from the relief of symptoms alone. But it was soon shown -that two manifestations of the disordered steroid metabolism-the excessively high 17-ketosteroid excretion in the urine and the excretion of the unusual steroid pregnantriol in the urine-are both promptly brought under control with return to normal values after cortisone administration. These points indicated clearly a gross disorder of adrenal cortical metabolism which was corrected by cortisone administration.Evidence then collected that production of hydro. cortisone by the adrenal cortex was seriously impaired in this disorder. Thus both Ely et al. (1953) and Bongiovanni et al. (1954) showed that the blood levels of circulating hydrocortisone were abnormally low in this disease, and soon afterwards Eberlein and Bongiovanni (1955a) found that the metabolites' of hydrocortisone in the urine were also present in abnormally small amount.To explain this syndrome of high 17-ketosteroid excretion, excessive pregnantriol excretion, and defective hydrocortisone production, Jailer (1953) postulated an enzymatic block of congenital origin in these subjects whereby they were unable to convert 17-hydroxyprogesterone, the probable normal precursor, to hydrocortisone itself. This would normally involve the action of a 21-hydroxylase, and this enzyme he suggested was lacking in congenital adrenal hyperplasia. This suggestion of Jailer's went far to explain the metabolic anomaly in the disease, and has found wide acceptance.But hints began to appear that the condition was not as homogeneous and the explanation not always so simple as Jailer's hypothesis would suggest. It had been recognized since 1951 (Shepard and Clausen, 1951;Wilkins et al., 1951) that a variant of the clinical disorder exists in which hypertension is a major feature. In 1955-Bongiovanni and Eberlein investigated one such patient and found strong evidence that the derangement of steroid metabolism was different in this patient from that previously encountered. There was, for instance, a relatively small pregnantriol excretion, but an excessively large excretion of a steroid, commonly called tetrahydro S, which differs from the usual metabolites of cortisone and hydrocortisone in having no hydroxyl group on the 11 carbon atom. In the urine of this patient no metabolites were found which contained the 1 1-hydroxyl grouping, and it appeared, therefore, that the defect in this variant of the disorder was not of the more usual 21-hydroxylase, but of a different enzyme, 1 1-hydrox...