Antenatal scanning detects urinary tract dilatation in 1-5% of pregnancies. Isolated hydronephrosis (i.e., that not due to secondary dilatation e.g. bladder dysfunction) is known to resolve or improve in most; some patients may, however, require intervention through deterioration of obstruction or renal function. Parents may enquire as to the possibility of resolution and time taken to achieve this. There are a number of classification systems available to the paediatric urologist. Here Jung et al. have looked at both the anteroposterior pelvic diameter (APPD) and Society for Fetal Urology (SFU) classification in regard to rate and time to resolution of hydronephrosis. The former system is quantitative and the latter, qualitative – SFU looks at shape, calyceal dilation and takes parenchymal thinning into account. The study is retrospective and included 831 patients diagnosed antenatally with hydronephrosis (January 1994 – June 2018). ‘Resolution’ of hydronephrosis was considered an APD <5mm and SFU grade I. Just over a half (54.3%) of renal units reached resolution during a median follow-up of nine months (range 1-133). Pyeloplasty was undertaken in 165 renal units (16.1%) that progressed to SFU grade IV and APPD larger than 20mm (grade 4) on repeated USS accompanied by a reduction in renal function (<40%) and t½ >20 minutes on MAG-3, implying obstruction. The remainder (29.6%) showed mild improvement or stability. SFU grades I through IV showed resolution of 81.7%, 65.6%, 37.6% and 5.2% respectively after 48 months of follow-up (this compares to APPD grades 1-4: 80%, 41.2%, 13.1% and 2.5%). In approximately half of cases, SFU and APPD grade were concordant, whereas 481 (46.8%) exhibited a grading discrepancy between the two systems (the majority, 406, having higher SFU than APPD grade). They found higher SFU grades (II to IV) categorised as APPD grade 1 demonstrated higher resolution rates (75.2% at 48 months) versus higher APPD grades (2-4) categorised as SFU grade I (53.2% at 48 months). This study provides useful data in resolution (54.3% at 48 months) using a strict definition of resolution (<5mm) and a further 30% showed some improvement / remained stable. Both APPD and SFU can nicely stratify hydronephrosis by severity, but APPD grades of 2, 3 and 4 are associated with a much lower cumulative resolution than the corresponding SFU grades (and therefore APPD is perhaps more accurate for predicting resolution of hydronephrosis in these patients).