Antenatal hydronephrosis is noted in approximately 1% of all pregnancies. Persistent renal obstruction may lead to progressive loss of renal function. Renal ultrasound (USS) is the preferred modality for diagnosis and monitoring of paediatric hydronephrosis as it is non-invasive, non-ionising, inexpensive and accessible. Increasing hydronephrosis on serial USS is often considered a risk factor for loss of renal function, while stable or decreasing hydronephrosis is considered a proxy for stable renal function. Jacobsen et al. retrospectively reviewed a cohort of children with severe (SFU grade 3 or 4) hydronephrosis. All had to have at least two renal USS and at least two diuretic renograms between January 2003 and July 2016. Imaging studies were used to quantify the degree of hydronephrosis and differential function. An independent radiologist reviewed imaging and assessed whether hydronephrosis was stable / improved / worsened (SFU grade and general appearances). A 5% change or greater in differential function was considered as significant. A total of 85 children were enrolled in the study. Interestingly, of the 75 children with stable or decreased hydronephrosis (by SFU grade or appearances) 65 (86.7%) had stable to improved differential function, while 10 (13.3%) had worsened differential function.Therefore, a cautionary tale is presented where some patients had worsening differential function despite stable or improved USS imaging. The authors suggest that this is important to consider when counselling parents regarding non-operative management of children with severe hydronephrosis who are followed without diuretic renography. However, one must consider that this study was retrospective and changes in differential function may be also due to the more healthy kidney developing further and thus displaying an increase in its uptake.