['Fracture rates in children are high. Cortical bone makes a major contribution to bone strength, determined by cortical geometry, mineralization and porosity. Of these, porosity remains least well explored. Since most cortical canals are part of an osteon, we examined osteons and their canals for age-related changes in numbers, size and shape in 87 iliac crest bone samples of subjects aged 0-25 years, using histomorphometry.', 'Three types of secondary osteons were identified: drifting, eccentric and concentric. 1. Drifting osteons predominated to the mid-teens, were large, asymmetrical, and had giant canals (remodeling space) with the resorption front drifting towards the marrow. The cause of drift remains unclear. Onset of formation appeared delayed, and commenced on the periosteum-facing surface. From the mid-teens numerical density of drifting osteons decreased, and so did porosity. 2. Eccentric osteons were smaller, more circular and had a small excentric canal; their numerical density gradually increased with age. 3. Concentric osteons (adult bone) were the smallest, most symmetrical osteons, had a small central canal, and markedly increased in numerical density from the mid-teens. Boys showed greater overall porosity and greater numerical density of drifting osteons, and later change to concentric osteons than girls. Whites had greater numerical density and greater areal density of resorption cavities than blacks.'] Is cortical porosity in children determined by age-dependent osteonal morphology?