Differential renal function (DRF) on MAG-3 scans is typically used in patients with pelvic ureteric junction obstruction (PUJ obstruction). Supranormal DRF is when the kidney with PUJO has higher DRF than the normal contralateral kidney. The authors in this study have analysed the significance of this and have also tried to predict factors that cause a supranormal DRF. They prospectively analysed 100 patients who underwent a pyeloplasty between 2012 and 2017. A DRF of >55% was defined by the researchers as supranormal. They correlated the DRF with the anteroposterior diameter (APD) and the ratio of pelvic and renal volumes (Vp/Vk). The final analysis included 78 patients with nine patients having a supranormal DRF with a prevalence of 11.5%. Preoperatively they found that any increase in APD increased the risk of supranormal DRF by 1.07 times with a 0.6% increase in DRF for every 1mm rise in APD which was statistically significant. They also noted that an APD >30mm being a reliable predictor with a sensitivity of 88.9%. The Vp/Vk ratio showed a 3.23 times increase in risk of supranormal DRF for every unit change although it could not significantly predict a percentage change in DRF. Functionally after repair the kidneys with supranormal function showed a significant fall in the DRF (57% vs. 51%) whereas the normal kidneys had a stable DRF (46% vs. 48%) after repair. These findings show the need for use of DRF with caution in patients who have supranormal scans. Patients with a supranormal DRF on MAG-3 should be counselled for early surgery and the potential for decline in renal function postoperatively.