Twenty patients with congenital adrenal hyperplasia due to 21-hydroxylase deficiency have been followed during a mean period of 68.4 months (from 19 to 120 months). Plasma 17 alpha-hydroxyprogesterone, delta 4-androstenedione and testosterone have been measured at regular intervals and correlated to growth and bone maturation. Satisfactory growth was obtained through repeated changes in the daily dose of oral hydrocortisone which varied from 58.7 +/- 37 mg/m2/day in infants to respectively 19.3 +/- 7.0 and 25.3 +/- 7.2 mg/m2/day in prepuberty and puberty. Fludrocortisone was added in 15 cases because of evidence of salt loss. delta 4-androstenedione and 17 alpha-hydroxyprogesterone are the best markers of adequate suppression of the pituitary-adrenal axis in both sexes. Testosterone can be used in girls and prepubertal boys. In some cases with low levels of androgens suggesting oversuppression, a reduced velocity of bone maturation was observed, particularly in young subjects.