Patients often come to the clinic to ask questions about hydronephrosis, so what exactly is hydronephrosis? This word comes from the English word “hydronephrosis”. In fact, there is a common misunderstanding about hydronephrosis. Hydronephrosis actually refers to the “accumulation of urine in the kidneys”, which is mainly due to various reasons that the urinary ducts do not flow smoothly, resulting in the accumulation of urine that cannot be excreted smoothly from the kidneys, manifested by the widening and enlargement of the urinary ducts inside and outside the kidneys. This phenomenon is somewhat similar to the “weir” caused by earthquakes and other geological disasters. Therefore, hydronephrosis is not a disease in itself, but a manifestation of several diseases. Persistent hydronephrosis may cause damage to kidney function. The presence of hydronephrosis requires further investigation of the cause and evaluation of kidney function. The causes of hydronephrosis are complex and can be broadly classified into congenital and acquired factors. Congenital factors include stricture of the urinary tract (narrowing of the pelvic-ureteral junction) and vesicoureteral reflux, which are usually detected early and even diagnosed during pregnancy. Acquired factors include urinary stones, benign and malignant ureteral tumors, ureteral strictures, chronic urinary retention in the elderly with prostatic hypertrophy and abdominopelvic surgery (retroperitoneal lymph node dissection, rectal cancer surgery, uterine surgery and other gynecological surgeries after radiotherapy), etc. In addition, diseases adjacent to the ureter, such as tumors compressing the ureter, may also lead to hydronephrosis.