Aims. Day-case knee and hip replacement, in which patients are discharged on the day of surgery, has been gaining popularity during the last two decades, and particularly since the COVID-19 pandemic. This systematic review presents the evidence comparing day-case to inpatient-stay surgery. Methods. A systematic literature search was performed of MEDLINE, Embase, and grey literature databases to include all studies which compare day-case with inpatient knee and hip replacement. Meta-analyses were performed where appropriate using a random effects model. The protocol was registered prospectively (PROSPERO CRD42023392811). Results. A total of 38 studies were included, with a total of 83,888 day-case procedures. The studies were predominantly from the USA and Canada, observational, and with a high risk of bias. Day-case patients were a mean of 2.08 years younger (95% CI 1.05 to 3.12), were more likely to be male (odds ratio (OR) 1.3 (95% CI 1.19 to 1.41)), and had a lower mean BMI and American Society of Anesthesiologists grades compared with inpatients. Overall, day-case surgery was associated with significantly lower odds of readmission (OR 0.83 (95% CI 0.73 to 0.96); p = 0.009), subsequent emergency department attendance (OR 0.62 (95% CI 0.48 to 0.79); p < 0.001), and complications (OR 0.7 (95% CI 0.55 to 0.89) p = 0.004), than inpatient surgery. There were no significant differences in the rates of reoperation or mortality. The overall rate of successful same-day discharge for day-case surgery was 85% (95% CI 81 to 88). Patient-reported outcome measures and cost-effectiveness were either equal or favoured day-case. Conclusion. Within the limitations of the literature, in particular the substantial risk of selection bias, the outcomes following day-case knee and hip replacement appear not to be inferior to those following an inpatient stay. The evidence is more robust for unicompartmental knee replacement (UKR) than for total knee replacement (TKR) or total hip replacement (THR). The rate of successful same-day discharge is highest in UKR, followed by TKR and comparatively lower in THR. Cite this article: Bone Joint J 2024;106-B(12):1385–1392