Abstract Context In primary hyperparathyroidism (PHPT) bone mineral density (BMD) is typically decreased in cortical bone and relatively preserved in trabecular bone. An increased fracture rate is observed however not only at peripheral sites but also at the spine, and fractures occur at higher BMD values than expected. We hypothesized that components of bone quality other than BMD are affected in PHPT as well. Objective To evaluate bone material properties using impact microindentation (IMI) in PHPT patients. Methods In this cross-sectional study, the Bone Material Strength index (BMSi) was measured by IMI at the midshaft of the tibia in 37 patients with PHPT (28 women), 11 of whom had prevalent fragility fractures, and 37 euparathyroid controls (28 women) matched for age, gender, and fragility fracture status. Results Mean age of PHPT patients and controls was 61.8 ± 13.3 and 61.0 ± 11.8 years, respectively, P = .77. Calcium and PTH levels were significantly higher in PHPT patients but BMD at the lumbar spine (0.92 ± 0.15 vs 0.89 ± 0.11, P = .37) and the femoral neck (0.70 ± 0.11 vs 0.67 ± 0.07, P = .15) were comparable between groups. BMSi however was significantly lower in PHPT patients than in controls (78.2 ± 5.7 vs 82.8 ± 4.5, P < .001). In addition, BMSi was significantly lower in 11 PHPT patients with fragility fractures than in the 26 PHPT patients without fragility fractures (74.7 ± 6.0 vs 79.6 ± 5.0, P = .015). Conclusion Our data indicate that bone material properties are altered in PHPT patients and most affected in those with prevalent fractures. IMI might be a valuable additional tool in the evaluation of bone fragility in patients with PHPT.