Background <p>Preoperative templating accuracy in cementless total hip arthroplasty (THA) may be influenced by patient-specific and radiographic factors, yet bone quality has rarely been considered. We hypothesized that bone mineral density (BMD), and particularly intertrochanteric BMD, would be associated with femoral stem size discrepancy.</p> Methods <p>In a retrospective cohort of 510 hips (467 patients) undergoing primary cementless THA, we compared templated and actual implant sizes for both femoral and acetabular components and assessed clinical and radiographic variables, including age, sex, weight, body mass index, BMD (T-score) at multiple proximal femoral sites, Dorr classification, and surgeon training level, using univariate and multivariable linear regression. Receiver operating characteristic analysis evaluated the discriminative performance of BMD for substantial stem size discrepancies.</p> Results <p>No factor was associated with acetabular cup size discrepancy. Age, weight, and intertrochanteric BMD were independently associated with femoral stem size discrepancy, with intertrochanteric BMD showing the strongest association among the variables examined (standardized β = −0.326). A T-score cutoff of − 0.9 was associated with discrepancies of at least three sizes larger than templated (area under the curve 0.782), and a cutoff of 0.2 with discrepancies of at least three sizes smaller (area under the curve 0.825).</p> Conclusions <p>Among the factors examined, intertrochanteric BMD showed the strongest association with femoral stem size discrepancy, although it explained only part of the overall variability. Preoperative assessment of intertrochanteric BMD may help anticipate substantial deviations in stem sizing and ensure that appropriately sized implants are available during surgery.</p>