Objective To evaluate the feasibility of keeping the consistency of bladder filing by a Bladderscan ( BS ) device during radiotherapy for rectal cancer. Methods The bladder volume was measured using a BS device and recorded. To verify the reliability of the BS measurement, the urinary output was determined by a measuring glass after complete urinary discharge. The bladder volume of 42 patients determined by the planning computed tomography ( CT) scans was used as the standard urinary volume. The bladder volume was measured using the BS device before radiotherapy every day. The chief complaint urinary volume was determined as the bladder volume when a patient felt a strong urge to urinate. The controlled urinary volume was determined as the urinary output intervened by the BS device to reach the standard urinary volume. Comparison was made by t test and data was assessed by Pearson correlation analysis. Results There was no significant difference in obtained urinary volume between the BS device and the planning CT scan ( P=0?84) . The urinary volume measured by the measuring glass was correlated with that measured by the BS device ( P=0?00 ) . The ability to hold urine in all patients was gradually weakened during treatment. The controlled urinary volume had a significantly smaller decrease after 1, 2, 3, 4, and 5 weeks of treatment than the chief complaint urinary volume ( 5% vs. 21%;6% vs. 20%;6% vs. 20%, 4% vs. 21%;11% vs. 26%;all P=0?00) . Conclusions Patients have a gradually decreased urinary volume during treatment. The intervention by the BS device gives a bladder volume close to that in the planning system, which results in a consistent bladder filling.