Background: Post-operative urinary retention (PUR) is associated with a risk of over-distension and permanent detrusor damage. The prevention of PUR by routine catheterization may increase the risk of urinary tract infection. Post-operative monitoring of the bladder volume is a reliable method in adults, but has not been evaluated for reliability in children. Methods: Forty-eight patients who, according to current clinical guidelines, required urinary bladder emptying were scanned before catheterization using the Bladderscan®. Volumes estimated with ultrasound were compared with volumes measured after emptying the bladder with a catheter. Bladder emptying was controlled by concomitant fluoroscopy in 20 children. Results: The mean difference between the ultrasound estimates and the catheter-emptied urine volumes was 4 ml [standard deviation (SD) = 25 ml] in 26 children above the age of 3 years and −18 ml (SD = 19 ml) in 22 younger children. In the subpopulation in whom complete bladder emptying could be confirmed by fluoroscopy (14 children; median age, 3 years; range, 1–11 years), the mean difference between the ultrasound estimates and the catheter-emptied volumes was −11 ml (SD = 24 ml). Conclusions: This study confirms agreement between the ultrasound scanner estimates of urinary bladder volume and the urine volume measured by emptying the bladder. Reliability was good in children above the age of 3 years. The volume was underestimated in younger children. Thus, routine monitoring of urinary bladder volume with an ultrasound scanner is a non-invasive, pain-free and reliable way of preventing over-distension of the urinary bladder in children after surgery and other procedures under general anaesthesia.