Aims. The purpose of this study was to compare outcomes of combined total joint arthroplasty (TJA) (total hip arthroplasty (THA) and total knee arthroplasty (TKA) performed during the same admission) versus bilateral THA, bilateral TKA, single THA, and single TKA. Combined TJAs performed on the same day were compared with those staged within the same admission episode. Patients and Methods. Data from the National (Nationwide) Inpatient Sample recorded between 2005 and 2014 were used for this retrospective cohort study. Postoperative in-hospital complications, total costs, and discharge destination were reviewed. Logistic and linear regression were used to perform the statistical analyses. p-values less than 0.05 were considered statistically significant. Results. Combined TJA was associated with increased risk of deep vein thrombosis, prosthetic joint infection, irrigation and debridement procedures, revision arthroplasty, length of stay (LOS), and in-hospital costs compared with bilateral THA, bilateral TKA, single THA, and single TKA. Combined TJA performed on separate days of the same admission showed no statistically significant differences when compared with same-day combined TJA, but trended towards decreased total costs and total complications despite increased LOS. Conclusion. Combined TJA is associated with increased in-hospital complications, LOS, and costs. We do not recommend performing combined TJA during the same hospital stay. Cite this article: Bone Joint J 2019;101-B:573–581