Detailed data on the physiological pattern of adrenocortical activity during normal growth are lacking. An established method to determine adrenocortical glucocorticoid secretion is the measurement of 24-h excretion rates of major urinary cortisol metabolites (C21). To test the hypothesis that the frequently reported higher cortisol secretion in men than in women develops during puberty, we examined C21 together with excretions of combined urinary free and conjugated cortisol (F_comb) in 400 healthy boys and girls aged 3-18yr using gas chromatography-mass spectrometry (GC-MS). Daily excretion rates of C21, F_comb, and body surface area (BSA)-corrected F_comb significantly increased with age in both sexes. In contrast, C21/BSA (µg m^-2 d^-1) declined from the age of 3-4yr to 7-8yr in boys and girls (P<0.01; e.g., in boys: from 3991±1167 to 3193±804), then increased in both sexes and finally became discordant after the age of 11-12yr with a further rise in males only (17-18 yr olds: males, 5275±1414; girls 3939±1586, P<0.01). This pattern was associated with the occurrence of a lower index for 5-reductase activity (allo-tetrahydrocortisol/tetrahydrocortisol) in females compared to males. Our results demonstrate dynamical changes in adrenocortical activity in healthy children resulting in an emerging sex dimorphism in cortisol secretion after age 11. The latter can be explained, at least partly, by diverging 5-reductase activities in boys and girls. F_comb, a frequently analyzed GC-MS parameter, proved not to reflect dynamical changes in cortisol secretion. In conclusion the varying metabolic need for cortisol during normal growth may have implications for future improvements in glucocorticoid replacement therapy.