The case of an infant girl with congenital adrenal hyperplasia due to 11β-hydroxylase deficiency is described. The diagnosis was made at the age of 1 week and the steroid excretion of the untreated patient was followed during the first year of life. Urinary THS reached very high levels only after 16α-hydroxypregnenolone had disappeared from the urine, suggesting an influence of the physiologically reduced 3β-hydroxysteroid dehydrogenase activity of infancy on the steroid pattern induced by the 11β-hydroxylase deficiency. The secretion rate of S was extremely high and that of F was low normal, while the secretion rates of DOC and B and the aldosterone excretion were within normal limits. This indicates an impairment of the 11β- hydroxylation of 17α-hydroxylated compounds only and is indirect evidence for the existence of two 11β-hydroxylating systems in the human adrenal cortex.