This study investigates sexual dimorphism in brain development during childhood and adolescence using longitudinal MRI data from 387 subjects (ages 3–27 years). The research highlights the importance of analyzing brain development trajectories over time rather than relying on group averages across broad age ranges. Using MRI, the study found that total cerebral volume peaks at 10.5 years in females and 14.5 years in males. White matter increases throughout the 24-year period, with males showing a steeper rate of increase during adolescence. Both cortical and subcortical gray matter volumes follow an inverted U-shaped trajectory, peaking 1–2 years earlier in females. These sexually dimorphic trajectories confirm the importance of longitudinal data in brain development studies and emphasize the need to consider sex matching in such research. The study compared brain development trajectories in males and females, focusing on gray and white matter volumes, as well as volumes of the caudate nucleus, lateral ventricles, and corpus callosum. The results showed that males generally had larger brain volumes, but after adjusting for total brain volume, significant differences in the shape and height of developmental trajectories were observed. For example, females had more frontal gray matter, while males had larger white matter volumes in certain regions. The study also found that the rate of increase in brain volume was higher in males, although this difference decreased as subjects reached their late teens and early twenties. The findings suggest that sexual dimorphism in brain development is age-dependent and that longitudinal studies are essential for understanding these differences. The study also highlights the complexity of brain development, noting that factors such as brain size, structure, and function are influenced by a variety of biological and environmental factors. The results have important implications for developmental neuropsychiatry, as they provide insights into the developmental pathways responsible for the increasing risk of psychiatric disorders during adolescence and the origin of sex-specific differences in symptoms.This study investigates sexual dimorphism in brain development during childhood and adolescence using longitudinal MRI data from 387 subjects (ages 3–27 years). The research highlights the importance of analyzing brain development trajectories over time rather than relying on group averages across broad age ranges. Using MRI, the study found that total cerebral volume peaks at 10.5 years in females and 14.5 years in males. White matter increases throughout the 24-year period, with males showing a steeper rate of increase during adolescence. Both cortical and subcortical gray matter volumes follow an inverted U-shaped trajectory, peaking 1–2 years earlier in females. These sexually dimorphic trajectories confirm the importance of longitudinal data in brain development studies and emphasize the need to consider sex matching in such research. The study compared brain development trajectories in males and females, focusing on gray and white matter volumes, as well as volumes of the caudate nucleus, lateral ventricles, and corpus callosum. The results showed that males generally had larger brain volumes, but after adjusting for total brain volume, significant differences in the shape and height of developmental trajectories were observed. For example, females had more frontal gray matter, while males had larger white matter volumes in certain regions. The study also found that the rate of increase in brain volume was higher in males, although this difference decreased as subjects reached their late teens and early twenties. The findings suggest that sexual dimorphism in brain development is age-dependent and that longitudinal studies are essential for understanding these differences. The study also highlights the complexity of brain development, noting that factors such as brain size, structure, and function are influenced by a variety of biological and environmental factors. The results have important implications for developmental neuropsychiatry, as they provide insights into the developmental pathways responsible for the increasing risk of psychiatric disorders during adolescence and the origin of sex-specific differences in symptoms.