Bone adapts to unaccustomed, high-impact loading but loses mechanosensitivity quickly. Short periods of military training (≤12 weeks) increase the density and size of the tibia in women. The effect of longer periods of military training, where the incidence of stress fracture is high, on tibial macrostructure and microarchitecture in women is unknown. This observational study recruited 51 women (age 19 to 30 years) at the start of 44 weeks of British Army Officer training. Tibial volumetric bone mineral density (vBMD), geometry, and microarchitecture were measured by high-resolution peripheral quantitative computed tomography (HRpQCT). Scans of the right tibial metaphysis (4% site) and diaphysis (30% site) were performed at weeks 1, 14, 28, and 44. Measures of whole-body areal bone mineral density (aBMD) were obtained using dual-energy X-ray absorptiometry (DXA). Blood samples were taken at weeks 1, 28, and 44, and were analyzed for markers of bone formation and resorption. Trabecular vBMD increased from week 1 to 44 at the 4% site (3.0%, p < .001). Cortical vBMD decreased from week 1 to 14 at the 30% site (−0.3%, p < .001). Trabecular area decreased at the 4% site (−0.4%);​ trabecular bone volume fraction (3.5%), cortical area (4.8%), and cortical thickness (4.0%) increased at the 4% site;​ and, cortical perimeter increased at the 30% site (0.5%) from week 1 to 44 (p ≤ .005). Trabecular number (3.5%) and thickness (2.1%) increased, and trabecular separation decreased (−3.1%), at the 4% site from week 1 to 44 (p < .001). Training increased failure load at the 30% site from week 1 to 44 (2.5%, p < .001). Training had no effect on aBMD or markers of bone formation or resorption. Tibial macrostructure and microarchitecture continued to adapt across 44 weeks of military training in young women. Temporal decreases in cortical density support a role of intracortical remodeling in the pathogenesis of stress fracture.