BACKGROUND: Post-anaesthesia care unit (PACU) admission must be well founded and the stay as short as possible without compromising patient safety. However, within the concept of fast-track surgery, studies are limited in addressing the question: why are patients staying in the PACU? METHODS: All patients operated with primary unilateral total hip or knee arthroplasty (THA or TKA) under spinal anaesthesia were included in this hypothesis-generating, prospective, observational cohort study during a 4-month period. Surgical technique, analgesia, and perioperative care were standardized. Well-defined PACU discharge criteria that had to be met on two successive assessments were evaluated every 15¿min until discharge. The primary outcome was time to meet PACU discharge criteria. Secondary outcomes were actual discharge time from the PACU, specific factors detaining patients in the PACU, and potential complications at the surgical ward at follow-up 24 h post-operatively. RESULTS: One hundred sixty-three patients were included in the final analysis (69 THA and 94 TKA). Time to meet PACU discharge criteria was [median (interquartile range)(95% confidence interval)]: 15¿min (15-15)(15-116) for THA and 15¿min (15-15)(15-75) for TKA. Actual discharge time from PACU was: 25¿min (20-35)(16-198) for THA and 25¿min (20-31)(15-107) for TKA. Reasons for not meeting PACU discharge criteria in 15¿min were mainly low oxygen saturation and pain. The short stay in the PACU did not impose complications at the surgical ward. CONCLUSION: The vast majority of patients (>¿85%) operated with THA and TKA under low-dose spinal anaesthesia may achieve pre-defined PACU discharge criteria in 15¿min. Large-scale studies should be performed to evaluate safety aspects after short PACU stay.