OBJECTIVE: Sex differences in disease susceptibility might be explained by sexual dimorphism in hypothalamic-pituitary-adrenal axis activity, which has been postulated to emerge during puberty. However, studies conducted thus far lacked an assessment of Tanner pubertal stage. This study aimed to assess the contribution of pubertal development to sexual dimorphism in cortisol production and metabolism. METHODS: Participants (n=218), were enrolled from a population-based Netherlands Twin Register. At the ages of 9, 12 and 17 years Tanner pubertal stage was assessed, and early-morning urine samples were collected. Cortisol metabolites were measured with GC-MS/MS, and ratios were calculated, representing cortisol metabolism enzyme activities, such as A-ring reductases, 11β-HSDs and CYP3A4. Cortisol production and metabolism parameters were compared between sexes for pre-pubertal (Tanner stage 1), early-pubertal (Tanner stage 2-3) and late-pubertal (Tanner stage 4-5) stages. RESULTS: Cortisol metabolite excretion rate decreased with pubertal maturation in both sexes, but did not significantly differ between sexes at any pubertal stage, although in girls a considerable decrease was observed between early- and late-pubertal stage (P<0.001). A-ring reductase activity was similar between sexes at pre- and early-pubertal stages, and was lower in girls than in boys at late-pubertal stage. Activities of 11β-HSDs were similar between sexes at pre-pubertal stage, and favored cortisone in girls at early- and late-pubertal stages. Cytochrome P450 3A4 activity did not differ between sexes. CONCLUSIONS: Prepubertally, sexes were similar in cortisol parameters. During puberty, as compared to boys, in girls the activities of A-ring reductases declined and the balance between 11β-HSDs progressively favored cortisone. Our findings suggest that the sexual dimorphism in cortisol may either be explained by rising concentrations of sex steroids or by puberty-induced changes in body composition.