Purpose: In recent years, there has been an increasing interest in simultaneous hip and knee arthroplasty compared to staged procedures. The aim of this study was to clarify some controversies and show the safety for the patients by comparing the transfusion rate, postoperative hemoglobin drop, length of stay (LOS), in-hospital complications, 30-day readmissions, and early functional outcome after simultaneous and staged hip and knee arthroplasty. Methods: We conducted a retrospective trial that included all patients who were undergoing primary TKA, THA, and UKA by one single surgeon in a high-volume arthroplasty center between 2017 and 2020 as a simultaneous or staged procedure. Staged bilateral arthroplasties were performed within 12 months and were stratified by the time between procedures. Data was acquired through the electronic files at the OCM. For functional outcome, we compared the ability of the patients to walk independently on the ward and the ability to walk a set of stairs alone which was daily recorded by the attending physiotherapist. Results: In total 305 patients were assessed for eligibility and included in this clinical trial. 145 patients were allocated to the staged arthroplasty group. That group was subdivided in a hip and a knee group, whereas the knee group was split in TKA and unicompartmental knee arthroplasty (UKA). The second staged procedure was performed within 12 months of the first procedure.160 patients were allocated to the simultaneous arthroplasty group. This group was also subdivided into a hip and knee group, whereas the knee group was split again into a TKA and UKA group. No statistical difference was found between the two groups regarding demographic data. Primary outcome measurements: There is no significant difference in transfusion rate or complication rate. Secondarily: No correlation was found between the Hb-drop and the functional outcome as well as the length of stay (LOS). Walking the stairs showed a significant difference in the knee group. Conclusion: We observed no significant differences in transfusion rate, in-hospital complications, as well as readmission rate. The early functional outcome showed no significant difference in mobility for all groups. Simultaneous arthroplasty for knee or hip is as safe as a staged procedure, with no higher risk for the patient and his outcome, in a high-volume center.