Background and purpose - Limited data exists on the implementation process and safety of discharge on the day of surgery after primary hip and knee arthroplasty in a multicenter setting. We report our study protocol on the investigation of the feasibility, safety, and socioeconomic aspects following discharge on day of surgery after hip and knee arthroplasty across 8 fast-track centers. Patients and methods - This is a study protocol for a prospective cohort study on discharge on day of surgery from the Center for Fast-track Hip and Knee Replacement. The collaboration includes 8 centers covering 40% of the primary hip and knee arthroplasty procedures undertaken in Denmark. All patients scheduled for surgery are screened for eligibility using well-defined inclusion and exclusion criteria. Eligible patients fulfilling discharge criteria will be discharged on day of surgery. We expect to screen 9,000 patients annually. Duration and outcome - Patients will be enrolled over a 3-year period from September 2022 and reporting of results will run continuously until December 2025. We shall report the proportion of eligible patients and patients discharged on day of surgery as well as limiting factors. Readmissions and complications within 30 days are recorded with real-time follow-up by research staff. Furthermore, patient-reported information on willingness to repeat discharge on day of surgery, contacts with the healthcare system, complications, and workability is registered 30 days postoperatively. EQ-5D, Oxford Knee Score, and Oxford Hip Score are completed preoperatively and after 3 months and 1 year. Finally, outcome data will be used in the development of a prediction model for successful discharge on the day of surgery.