Hydronephrosis of 20mm is very serious. Under clinical physiological conditions, hydronephrosis will not exceed 10mm. Once it exceeds 20mm, obstruction, such as ureteral kidney stone, tumor or retroperitoneal fibrosis, is generally considered, and active treatment is recommended. Because hydronephrosis will compress the normal kidney tissue and make the kidney cortex thinner, in serious cases, pus in the kidney pelvis or impaired kidney function and kidney failure can occur. For urinary tract infections, active anti-inflammatory treatment is recommended for at least two weeks. If it is caused by stones, extracorporeal shock wave lithotripsy or surgical stone extraction is recommended. If the hydronephrosis has caused complete loss of kidney function, it is recommended to actively perform surgery to remove the focal kidney.