To date, few knowledge is available about safety and effectiveness of one-staged combined hip and knee arthroplasty. The aim of our study was to evaluate, in a comparative fashion, complications and outcomes in patients who underwent one-staged hip and knee arthroplasty. Forty-two patients were enrolled and allocated into two groups of 21 patients each: one-staged hip and knee arthroplasty (group A) and two-staged hip and knee arthroplasty (group B). The follow-up averaged 50.2 months. Postoperative complications and implant survivorship were assessed prospectively. Outcomes were evaluated with Harris Hip score (HSS), Western Ontario Mc-Ministry score for the hip (h-WOMAC), Knee Society score (KSS), and Western Ontario Mc-Ministry knee score (k-WOMAC). Hip and knee range of motion (ROM) were measured both preoperatively and at the last follow-up. Two (9.5%) patients in group A and three (14.3%) patients in group B developed complications (P = 0.8). Although a significant decrease in postoperative haemoglobin (Hgb) values was found in group A patients during the hospital stay, no differences in blood transfusions were found (P = 0.8). No significant differences were found comparing clinical-functional outcomes between the two groups, while a significant reduction of hospital length of stay was shown in group A patients. One-staged combined hip and knee arthroplasty could be considered in patients with co-existing severe hip and knee osteoarthritis, providing similar complications and mid-term outcomes of two-staged procedures. However, the reproducibility safety and reliability of these procedures should be confirmed in prospective comparative randomised trials with more numerous patients. Retrospectively registered