Structural adaptations of bone to changing mechanical loads have recently been documented during adolescence. However, little is known about how bone adapts structurally during the earlier years. Using a longitudinal observational design spanning 6 years of growth (age range 4 to 12 years), we investigated associations between everyday physical activity and hip geometry in a cohort of healthy Midwestern children (n=468). Femoral neck (FN) cross sectional area (CSA, cm(2)) and FN section modulus (Z, cm(3)) were used to describe hip geometry. CSA and Z, indices of axial and bending strength, were assessed using dual-energy X-ray absorptiometry (DXA) scans and the hip structure analysis (HSA) program. Moderate and vigorous physical activity (MVPA) was assessed using accelerometry-based activity monitors and calculated as the number of minutes > or =3000 accelerometry movement counts. Data were analyzed using multilevel (random- and fixed-effects) regression models with adjustment for age (year), height (cm), and weight (kg) or lean mass (kg). For boys and girls, MVPA was a positive independent predictor of CSA and Z (p<0.05). On average, children who participated in 40 min of MVPA per day would be expected to have 3% to 5% greater CSA and Z than peers participating in 10 min of MVPA per day. Ten-minute increases in daily MVPA had similar effects on CSA in girls and Z in boys as did each additional 1 kg of body weight. When lean mass was substituted for weight, MVPA continued to be a positive independent predictor of CSA and Z for boys, but not girls. This study demonstrates that everyday amounts of physical activity in children are associated with indices of FN bone strength during childhood. Differences in lean mass mediate associations between physical activity and hip geometry in girls, but only somewhat in boys. These results suggest that physical activity is an important contributor to bone strength prior to adolescence and that increasing levels of physical activity during childhood are likely to enhance optimal bone strength.