Endoprosthetic reconstructions of the hip and knee are currently reimbursed as primary hip and knee arthroplasty according to Current Procedural Terminology ( CPT ) coding guidelines. The purpose of this study was to compare hospital resources consumed by patients undergoing endoprosthetic reconstruction with those consumed by patients undergoing primary arthroplasty. The authors' hypothesis was that the former group carries more comorbidities, experiences longer length of stay (LOS), and has greater resource consumption. A retrospective review was performed of 61 patients undergoing endoprosthetic reconstruction and 745 patients undergoing primary hip or knee arthroplasty between 2015 and 2018 at a single institution. Demographic, clinical, and financial data were compared. The Charlson Comorbidity Index (CCI) was used to measure patients' health status and identify comorbidities associated with prolonged LOS through linear regression analysis. Patients who underwent endoprosthetic reconstruction had a greater than 3.5 times average LOS compared with primary arthroplasty patients: 10.81 days vs 2.94 days ( P <.01). They demonstrated a higher mean CCI, higher rates of malignancy and pulmonary disease, and a wider age range. Their mean cost of care totaled $73,730.29, compared with $24,940.84 imposed by primary arthroplasty patients ( P <.01). Significant predictors of LOS were malignancy status (metastatic or localized) and age younger than 50 years, with increased LOS being associated with increased cost. Patients undergoing endoprosthetic reconstruction of the hip and knee represent a fundamentally different patient population than primary arthroplasty patients based on comorbidities, variability in health status, and surgical indications. They have higher comorbidity scores and longer hospitalizations and consume more financial resources than primary arthroplasty patients. [ Orthopedics . 2021;44(1):e73–e79.]