Hydronephrosis is commonly caused by dysfunction of the pelviureteric junction (PUJ). Other causes, which are usually associated with hydroureter as well hydronephrosis, include vesicoureteric reflux, vesicoureteric obstruction and bladder pathology. Common causes of PUJ dysfunction include intrinsic stenosis and aberrant lower pole vessels. Surgery, usually an Anderson–Hynes type pyeloplasty, is indicated in patients with pain, infection or haematuria. Surgery may be performed open or laparoscopically. Asymptomatic patients, often detected during antenatal screening, require a baseline MAG3 scan and serial monitoring with ultrasound scanning. Surgery is indicated in this cohort if there is increasing hydronephrosis with an anterior posterior diameter (APD) greater than 3 cm, function below 40% or a drop in function of more than 10% on serial MAG3 scans.