The measurement of urinary steroid metabolites before and after ACTH stimulation was used to determine the extent of the 21-hydroxylase defect present in the cortisol pathway of 18 patients with congenital adrenal hyperplasia, who were receiving steroid therapy. The results indicate that the degree of 21-hydroxylase deficiency in the cortisol pathway is directly related to the two clinical variants of the disease, salt-losing and non-salt-losing.