Advances in anaesthetic technology and orthopaedic practice are enabling same day discharge (SDD) following total hip and knee arthroplasty (THA and TKA). SDD is also an appealing strategy for resource constrained health systems to facilitate THA and TKA for increasing numbers of patients disabled by endstage hip and knee arthritis. Previous research of SDD has aimed to identify suitable patients for this endeavour, but no consensus is available regarding demographics, comorbidities or preoperative scoring systems. One limitation of prior research is that data derives from single ‘centres of excellence’ which may not be generalisable, particularly to smaller hospitals across a large health care system. In this context our goals were two-fold: 1) Assess the safety of SDD across all centres performing THA and TKA in Ontario and 2) Identify patient, surgeon and institutional variables that were significantly associated with failure of SDD. We conducted a population-based, retrospective cohort study of all patients undergoing primary THA and TKA in Ontario between 2016 to 2021. Data was extracted from the ICES database and previously validated algorithms were utilised to identify patients, covariates and outcomes. Inclusion criteria included patients undergoing primary total hip or knee arthroplasty treated by all surgeons and hospitals in Ontario. We excluded revision arthroplasties and other arthroplasty operations (ex. partial knee replacements). “Failure” of SDD was defined as the inability to discharge the patient on the same day / admission to hospital following originally planned SDD. Of 58,120 THAs completed between 2016 and 2021, 3,380 patients were planned for SDD. There were no differences in medical complications (DVT/PE, MI, pneumonia) between those planned SDD versus inpatients. The proportion of patients planned for SDD increased from 0.5% in 2016 to 33% in 2021. Of those planned for SDD, 2981 (88.4%) were successful and 393 (11.6%) failed/could not be discharged the same day. Predictors for failure were Charlson index (p=0.042), obesity (p=0.002), female gender (p<0.001), PUD (p=0.011), frailty (p<0.002), hypertension (p=0.022), use of general anaesthetic (p<0.001) and surgical complications (p=0.006). Of 82,646 TKAs completed between 2016 and 2021, 2,776 patients were planned for SDD. Similar to THA, there were no differences in medical complications (DVT/PE, MI, pneumonia) or mortality within 30 days between those planned SDD versus inpatients following TKA. The proportion of patients planned for SDD increased from 0% in 2016 to 27% in 2021. Of those planned for SDD, 2462 (88.7%) were successful and 314 (11.3%) failed. Once again predictors of failure included Charlson index (p=0.037), frailty (p<0.01), female gender (p=0.019) and use of general anaesthetic (p<0.001). Interestingly for both THAs and TKAs, those with successful SDD were more likely to have an unplanned ED visit within 30 days of surgery (p<0.001). SDD following THA and TKA in Ontario increased from <1% in 2016 to nearly a third of patients in 2021. The early experience of SDD following these procedures in Ontario appears to be safe without and increased risk of medical complications and mortality compared to inpatients. Failure of SDD only occurred in about 10% of patients. Our findings that female gender, obesity, medical comorbidity and general anaesthetic delaying discharge can be helpful in planning for SDD success. Interestingly, the increased ED admission in those with successful SDD suggests a failure to detect early complications, negate patient anxieties or deal with conditions suitable for primary care and is an area for improvement going forward. Future research should assess low fidelity testing including clinical examination findings to help predict SDD success.