Limb-salvaging hemipelvectomy surgeries involving allograft or custom prosthesis reconstruction require high quality remaining pelvic bone for adequate device fixation. Modeling studies of custom pelvis prosthesis designs typically mirror contralateral pelvic bone material properties to the ipsilateral pelvis. However, the extent of bone material property and geometric symmetry, and thus the appropriateness of mirroring, remains unknown and should be considered when designing or analyzing the performance of pelvic prostheses. This study investigates preoperative differences between ipsilateral and contralateral pelvic bone for patients with a pelvic sarcoma. Computed tomography (CT) data were obtained retrospectively from eight patients with a pelvic sarcoma. Subject-specific computational models of the pelvic bones were constructed from the CT data. Bilateral asymmetry of bone material properties and cross-sectional areas between the ipsilateral and contralateral hemipelvis were quantified at points adjacent to the pelvic sarcoma. Large bilateral asymmetry (>20%) in trabecular but not cortical bone density was observed within 20 mm of the tumor location. Differences in trabecular bone density typically declined with increased distance from the tumor. The greatest bilateral difference in cross-sectional area occurred within 10 mm of the tumor boundary for three patients and within 40 mm from the tumor site for four patients. Our results suggest that pelvic sarcomas can cause significant bilateral asymmetries in trabecular bone density for patients with a pelvic sarcoma. These differences should be taken into account when designing custom implants for this patient population.