Introduction: It is recommended that the bladder be filled at least twice to perform a standard urodynamic study. However, the procedure is time-consuming and distressing for both the patient and the professional. The impact of the number of fillings on urodynamic parameters in children with normal urodynamic results and those with detrusor instability is presented in this study. Patients and Methods: Urodynamic data from 70 children with detrusor instability and 42 with normal urodynamic parameters who had undergone 3 consecutive fillings in a single session during a 2-year period were reviewed retrospectively. The variable parameters that were compared between the 2 groups were the mean bladder capacity ratio, maximum filling pressure and the number of contractions during the filling phase. Results: In the normal group, no difference was found in bladder capacities between the 3 fillings, but there was a significant decrease in the maximum bladder filling pressures in the subsequent fillings. In children with detrusor instability, the number of contractions in the 1st filling was significantly higher than those in the subsequent fillings. Also in this group, a significant difference in the mean bladder capacities between fillings was noted as well as a decrease in the maximum bladder filling pressures in contrast to normal bladders. Conclusion: In patients with suspected detrusor instability the bladder should be filled at least twice for a reliable urodynamic assessment. Two normal fillings without contractions, with normal capacity and normal maximum bladder filling pressure are highly suggestive of a normal 3rd filling. Copyright (c) 2005 S. Karger AG, Basel.