To determine whether cortisone acetate given as a single daily dose would be as effective as the same amount divided into three doses in suppressing adrenal androgen secretion in congenital adrenal hyperplasia, we studied three patients with the salt-losing variety, who were judged to have been adequately treated during the previous year. Each patient was receiving cortisone acetate, 20.6 to 22.6 mg/m2 body surface area per day, divided into three doses, and 9 alpha-fluorohydrocortisone, 0.05 to 0.1 mg/day. Their spontaneous and adrenocorticotropic hormone-stimulated blood concentrations of 17 alpha-hydroxyprogesterone, androstenedione, and testosterone were measured initially and were normal. Cortisone acetate was then given once a day in the same total daily dose. After 3 months, plasma adrenocorticotropic hormone concentration was increased in all three patients, and in two of them the plasma levels of 17 alpha-hydroxyprogesterone and androstenedione were also increased. In the third patient, after 7 months of once-daily therapy, plasma levels of 17 alpha-hydroxyprogesterone and androstenedione were increased. We conclude that a physiologic replacement dosage of cortisone acetate given once daily is not effective in controlling excessive adrenal androgen secretion in congenital adrenal hyperplasia.