This study aimed to investigate whether there was an association between an unanticipated prolonged post-anaesthesia care unit (PACU) length of stay and early postoperative deterioration, as defined as the need for a rapid response team activation, within the first seven days of surgery. We conducted a single-centre retrospective cohort study of adult surgical patients, who stayed at least one night in hospital, and were not admitted to critical care immediately postoperatively, between 1 July 2017 and 30 June 2019. A total of 11,885 cases were analysed. PACU length of stay was significantly associated with rapid response team activation on both univariate (odds ratio (OR) per increment 1.57, 95% confidence intervals (CI) 1.45 to 1.69, P < 0.001) and multivariate analysis (OR per increment 1.41, 95% CI 1.28 to 1.55, P < 0.001). Patients who stayed less than one hour were at low risk of deterioration (absolute risk 3.7%). In patients staying longer than one hour, the absolute increase in risk was small but observable within six hours of PACU discharge. Compar\ed to a one-hour length of stay, a five-hour stay had a relative risk of 4.9 (95% CI 3.7 to 6.1). Other factors associated with rapid response team activation included non-elective surgery (OR 1.78, P < 0.001) and theatre length of stay (OR per increment 1.61, P < 0.001). PACU length of stay was also independently associated with predefined complications and unplanned intensive care unit admission postoperatively. In our cohort, an unanticipated prolonged PACU length of stay of over one hour was associated with an increased incidence of rapid response team activation in the first seven days postoperatively.