Six patients with congenital virilizing adrenal hyperplasia were evaluated on single- and multiple-doseprednisone schedules. Each of the treatment periods was for one month. Patients were evaluated by 24-hour urinary excretion of 17-ketosteroids and pregnanetriol, as well as 0900 plasma concentrations of 17-hydroxyprogesterone, progesterone, and testosterone. By the criteria of urinary excretion of KS and PNT appropriate for chronologic age, three of the six patients were adequately controlled on prednisone given once a day. Prednisone administered twice daily at 12-hourly intervals either in equally divided doses or with a large dose in the evening, however, resulted in adequate suppression in all patients. Because of the marked diurnal variation of plasma 17-OHP, the time of day that the sample is drawn is critical. Afternoon samples are often misleadingly low. Plasma 17-OHP concentration may reflect escape from therapeutic control sooner than urinary KS and PNT excretion. There was no correlation between 17-OHP and P values. Plasma concentration of T was not a reliable indicator of good control, since T values were often at prepubertal levels when urinary KS and PNT were elevated.