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Background: Untreated postoperative urinary retention can result in permanent lower urinary tract dysfunction and can be prevented by timely bladder catheterization. The author hypothesized that the incidence of postoperative bladder catheterization can be decreased by using the patient's own maximum bladder capacity (MBC) instead of a fixed bladder volume of 500 ml as a threshold for catheterization. Methods: Randomized parallel-arm and single-blinded comparative effectiveness trial conducted in 1,840 surgical patients, operated under general or spinal anesthesia without an indwelling urinary catheter. Patients were randomized to either use their individual MBC (index) or a fixed bladder volume of 500 ml (control) as a threshold for postoperative bladder catheterization. Preoperatively, the MBC was determined at home by voiding in a calibrated bowl. All other bladder volumes were measured by ultrasound. Postoperatively, bladder catheterization was performed when spontaneous voiding was impossible, and the ultrasound measurement exceeded the threshold for the group in which the patient was randomized (500 or MBC). The primary outcome was the incidence of bladder catheterization. Results: The average MBC in the control group was 582 ml (199 ml) and in the index group 611 ml (+/- 209 ml). The incidence of catheterization decreased from 11.8% (107 of 909 patients) in the control group to 8.6% (80 of 931) in the index group (relative risk 0.73, 95% CI 0.55 to 0.96, P = 0.025). There were no adverse events in either group. Conclusions: In patients undergoing surgery under general or spinal anesthesia using the MBC rather than a fixed 500 ml threshold for bladder catheterization is a safe approach that significantly reduces the incidence of postoperative bladder catheterizations.