Background: Fractures in children are common and increasing in incidence. Previous research into the effects of bone mass and other determinants of fracture risk in children has been mainly of a cross-sectional or case-control study design, and there is a need for detailed prospective data to explore the specific roles of volumetric bone density and bone size. Aims: 1.To carry out a systematic review and meta-analysis of the previously published evidence for an association between bone mass and fractures in children; 2. To investigate the confounding structure of childhood bone mass using a contemporary geographical birth cohort; 3. To prospectively investigate volumetric bone density and bone size as determinants of childhood fractures; 4. To investigate other determinants of childhood fracture risk that may act independently of bone mass. Methods: A dual energy X-ray absorptiometer (DXA) scan was performed in 5933 children at aged 9.8 years from the Avon Longitudinal Study of Parents and Children. Reported fractures were collected over the next 24 months. Data had been previously collected on variables such as gender and socio-economic status. Variables were analysed to see if they affected fracture risk by an action that was independent of bone mass. A novel method of using humerus data from total body DXA scans was developed to allow exploration of biomechanical strength. Data were analysed using t-tests and multivariable regression. Results: Of the 5933 children, 527 (8.9%) reported at least one fracture over the two-year follow-up period. Per standard deviation (SD) decrease in estimated volumetric density, fracture risk in children approximately doubled over the following two years (OR 1.96, 95%CI 1.27 to 3.01, P=0.002). Per SD decrease in bone size relative to body size, fracture risk increased by 62% (OR 1.62, 95%CI 1.23 to 2.15). Family size, gender, ethnicity, physical activity and birth-weight were risk factors for fractures that were independent of bone mass. Conclusions: Bone fragility as measured by volumetric bone density and bone size relative to body size, are determinants of fracture risk in children. However, other risk factors acting via increased exposure to injuries or via the mechanism of injury are also important.