In the Active‐Controlled Fracture Study in Postmenopausal Women With Osteoporosis at High Risk (ARCH) clinical trial (NCT01631214), 1 year of romosozumab followed by alendronate reduced the risk of vertebral and nonvertebral fractures compared to alendronate alone in women with prevalent fracture. We performed post hoc analyses of data from patients in ARCH (romosozumab, n = 1739;​ alendronate, n = 1726) who had a baseline BMD measurement and received at least one open‐label alendronate dose. We evaluated 1‐year mean BMD and corresponding T‐score changes;​ proportions of patients achieving T‐scores > −2.5 at the total hip (TH), femoral neck (FN), and lumbar spine (LS);​ and group differences in fracture rates after 12 months, while all participants were on alendronate. Subsequently, we investigated the relationship between T‐scores achieved at the TH, FN, and LS at 12 months and subsequent fracture incidence. At 1 year, mean change from baseline in TH BMD was 6.3% (T‐score change 0.31) with romosozumab versus 2.9% (T‐score change 0.15) with alendronate (p < .001). The proportion of patients with TH T‐score > −2.5 increased from 34% at baseline to 55% after 1 year of romosozumab and from 32% at baseline to 44% after 1 year of alendronate. Compared with patients receiving alendronate in year 1, those receiving romosozumab had a 75% reduction in new or worsening vertebral fracture (p < .001) in year 2, and a 19% reduction in nonvertebral fracture (p = .120) and 40% reduction in hip fracture (p = .041) during the open‐label period. TH and FN T‐scores achieved at month 12 were associated with subsequent nonvertebral and vertebral fracture rates and the relationships were independent of treatment received. LS T‐score at 12 months was associated with vertebral but not nonvertebral fracture risk. We conclude that 1 year of romosozumab leads to larger BMD gains versus alendronate, and that the T‐score achieved with either therapy is related to subsequent fracture risk. These data support the use of T‐score as a therapeutic target for patients with osteoporosis.