The efficacy of 3 years of annual intravenous administration of zoledronic acid (ZOL) in reducing vertebral and nonvertebral fractures in postmenopausal osteoporosis has been shown by the HORIZON pivotal fracture trial. Histomorphometric analysis of transiliac bone biopsies from the HORIZON participants revealed significantly improved trabecular architecture and reduced bone remodeling for the ZOL-treated versus placebo-treated patients. The aim of our study was to evaluate the cancellous and cortical bone mineralization density distribution (BMDD) in these biopsies by quantitative backscattered electron imaging (qBEI). The study cohort comprised 82 patients on active treatment (ZOL, yearly doses of 5 mg) and 70 treated with placebo, and all received adequate Ca and VitD supplementation. Comparison of ZOL-treated versus placebo-treated cancellous (Cn.) and cortical (Ct.) BMDD-derived variables resulted in significantly higher average (Cn.CaMean + 3.2%, Ct.CaMean + 2.7%) and mode calcium concentrations (Cn.CaPeak + 2.1%, Ct.CaPeak + 1.5%), increased percentages of highly mineralized bone areas (Cn.CaHigh + 64%, Ct.CaHigh + 31%), lower heterogeneity of mineralization (Cn.CaWidth −14%, Ct.CaWidth −13%), and decreased percentages of low mineralized bone areas (Cn.CaLow −22%, Ct.CaLow −26%) versus placebo (all p < 0.001). Cn. BMDD from the patients on active treatment also revealed a statistically significant shift to higher Ca concentrations when compared to a historical normal reference BMDD. These differences in BMDD from ZOL patients compared to the other groups were in line with the correlation of BMDD variables with previously determined cancellous mineralizing surface per bone surface (Cn. MS/BS, a primary histomorphometric index for bone turnover), showing that those with lower Cn. MS/BS had a higher degree of bone matrix mineralization. However, the differences in BMDD variables between the study groups remained when adjusted for Cn. MS/BS, suggesting that other factors in addition to reduced bone turnover might contribute to the higher bone matrix mineralization after ZOL treatment. © 2013 American Society for Bone and Mineral Research.