The value of determining concentrations of serum 17-α-hydroxyprogesterone, progesterone, estradiol, and testosterone in the diagnosis and management of congenital adrenal hyperplasia was studied. The values recorded were compared with established criteria of control: growth rate, advancement of bone age, degree of virilization, and 24-hour urine excretion of 17-ketosteroids and pregnanetriol. Serum 17-OHP concentrations were diagnostic in five new patients with CAH; they ranged from 100-312 ng/ml (50 to 450-fold above normal); serum progesterone concentrations ranged from 7-24 ng/ml (2 to 50-fold above normal). In the long-term management no one serum steroid concentration alone could be relied upon to determine adequacy of control. It appears that several variables, including diurnal variation, timing of the sample in relation to the last dose of a glucocorticoid, and chronic suppression of the hypothalamic-pituitary-adrenal axis, in addition to the degree of control, may affect a single serum steroid determination. When an acceptable range of normal in CAH for concentration of serum 17-OPH was broadened to 4 ng/ml and for progesterone to 1 ng/ml, there was a fairly good correlation of serum steroid concentrations with degree of control. © 1974 The C. V. Mosby Company.