ABSTRACT Purpose Reflux grade predicts resolution and influences followup and treatment. Bladder pressure at the onset of reflux may reflect the ureterovesical junction competence. By combining simultaneous cystometry with nuclear cystography (nuclear cystometrogram) we determined bladder pressure at the onset of reflux. Materials and Methods Nuclear cystometrograms were performed in 40 children to determine bladder pressure at the onset of reflux, reflux volume and bladder capacity at the onset of reflux. The effect of bladder pressure and capacity at the onset of reflux was assessed with a reflux pressure volume index. Results A weak inverse correlation existed between bladder pressure at the onset of reflux and a normalized volume of reflux (r = −0.32, p = 0.018). A stronger correlation existed between reflux pressure and bladder volume at reflux (r = 0.47, p = 0.0002). An inverse correlation of the reflux pressure-volume index and reflux volume was significant (r = −0.49, p = 0.0002) as was the correlation between bladder volume at reflux and amount of reflux (r = −0.47, p = 0.0003). The reflux pressure-volume index for patients with persistent reflux at followup (median 1.89) was significantly smaller than that for those without reflux (median = 8.71, p = 0.02). A difference existed in normalized bladder volume at reflux between resolved and persistent reflux (p = 0.003). No difference was detected with respect to reflux volume and resolution (p = 0.738). Conclusions A nuclear cystometrogram is no more invasive than a standard cystogram and permits determination of bladder pressure at the onset of reflux. Factors such as reflux pressure and bladder volume at reflux provide additional characterization of the ureterovesical junction and may help determine the prognosis for reflux resolution.