Context: In healthy boys, fractures result from trauma of various severity, suggesting contribution of an intrinsic biomechanical fragility. Objectives: Our objective was to characterize bone mineral mass, microstructure, and strength in boys with and without fractures. Participants and Design: We followed 176 healthy boys from 7.4 ± 0.5 to 15.2 ± 0.5 (mean ± sd) yr of age. Outcomes: Areal (a) bone mineral density (BMD) was measured by dual-energy x-ray absorptiometry at radius metaphysis and diaphysis, total hip, femoral neck and diaphysis, and L2-L4 vertebrae. Volumetric (v) BMD and microstructure were assessed by high-resolution peripheral computerized tomography at both distal tibia and radius. Bone strength was evaluated by micro-finite element analysis. Results: A total of 156 fractures were recorded in 87 of 176 boys with peak incidence between 10 and 13 yr. At 7.4 yr, subjects with fractures had lower aBMD in all sites and at 15.2 yr in femoral and spinal, but not in radius, sites. At that age, boys with fractures displayed lower trabecular (Tb) vBMD (P = 0.029) and number (P = 0.040), stiffness (P = 0.024), and failure load (P = 0.016) at distal tibia, but not distal radius. Odds ratios of fracture risk per 1 sd decrease were 1.80 (P = 0.006) for femoral neck aBMD and 1.46 (P = 0.038) for distal tibia Tb vBMD, 1.59 (P = 0.031) for Tb number, 1.53 (P = 0.072) for stiffness, and 1.60 (P = 0.056) for failure load. Conclusion: In a homogeneous cohort of healthy boys, fractures recorded until 15.2±05 yr of age were associated with lower femoral neck aBMD and with lower distal tibia trabecular vBMD and number, stiffness and failure load. These deficits in bone mineral mass, microstructure and strength could contribute to the occurrence of fractures during growth.