The onset of walking in early childhood results in exposure of the lower limb to substantial forces from weight bearing activity which ultimately contribute to adult bone strength. Relationships between gross motor score (GMS), at 18 months and bone outcomes measured at age 17 years were examined in 2327 participants in the Avon Longitudinal Study of Parents and Children (ALSPAC). Higher GMS indicated greater motor competence in weight bearing activities. Total hip bone mineral density (BMD) and hip cross-sectional moment of inertia (CSMI) were assessed from dual-energy X-ray absorptiometry (DXA). Bone measures including cortical bone mineral content (BMC), periosteal circumference (PC), cortical thickness (CT), cortical bone area (CBA), cortical BMD (BMDC ) and cross sectional moment of inertia (CSMI) were assessed by peripheral quantitative computed tomography (pQCT) at 50% distal-proximal length. Before adjustment, GMS was associated with hip BMD, CSMI and tibia BMC, PC, CT, CBA and CSMI (all P < 0.001) but not BMDC (P > 0.25). Strongest associations (standardised regression coefficients with 95% CI) were between GMS and hip BMD [0.086 (0.067, 0.105)] and tibia BMC [0.105 (0.089, 0.121)]. With the exception of hip BMD, larger regression coefficients were observed in males (sex interactions all P < 0.05). Adjustment for lean mass resulted in substantial attenuation of regression coefficients, suggesting associations between impaired motor competence and subsequent bone development are partly mediated by alterations in body composition. In conclusion, impaired motor competence in childhood is associated with lower adolescent bone strength, and may represent a risk factor for subsequent osteoporosis. This article is protected by copyright. All rights reserved.