Background: Post-operative urinary retention (POUR) is most accurately determined by using ultrasound to measure bladder volume. The aim of this study was to define the risk factors of POUR in the recovery room in hospitalised patients. Methods: An ultrasound-determined bladder volume ≥400 ml at arrival in the recovery room was used to define POUR. Multivariate regression analysis was used to identify patient and system factors linked to POUR in 773 consecutive hospitalised patients who had undergone orthopaedic, abdominal, gynaecological or plastic surgery without an indwelling urinary catheter. Results: We found the incidence of POUR to be 13%. The lack of pre-operative voiding, use of regional anaesthesia, anaesthesia time >2 h and emergency surgery were all independent risk factors for POUR. Conclusions: The detected incidence of POUR at arrival in the recovery room was rather high but had easily identifiable risk factors. We recommend pre-operative voiding whenever possible. Routine bladder scanning at arrival in the recovery room should be considered, especially after spinal anaesthesia, emergency surgery or when the anaesthesia time exceeds 2 h.