In recent times, the problematic milk powder has triggered parents to bring their children to the hospital for screening. During our screening, we found that some children did not have kidney stones and crystals as they were feared beforehand, but had other problems such as hydronephrosis, malformation, and renal agenesis. In recent times, there have been more inquiries about hydronephrosis, and what I know about hydronephrosis is presented below. In recent years, during pregnancy checkups, it has been found that the fetus may have a separated pelvis and ureter, which means a small amount of hydrocele, but most of the children are born with the hydrocele reduced or disappeared. There are also some children in whom this fluid persists for some time after birth. This may be due to the relatively narrow ureter in infants and children, which causes a small amount of urine accumulation in the renal pelvis. Most children have lighter effusion, under 10mm, and the child usually has no symptoms and urinates and eats normally. Regular follow-up after birth or discovery is usually recommended to compare whether the effusion has worsened or decreased. At the beginning, because parents are more worried or are not clear about the cause of hydrocele formation, they can review more frequently, such as 1~3 months after birth, and if there is no significant increase, the interval of review can be extended. Generally speaking, as long as the hydrocele is no longer aggravated and the child does not have abnormal urination or recurrent urinary tract infection symptoms, the child can be observed regularly and does not need special treatment. It is enough to feed more water appropriately and maintain sufficient urine output. If the fluid accumulation is found to be aggravated, the kidney volume is increased, or even the upper ureter is dilated, it is mentioned that there is a stenosis of the pelvic ureteral connection or ureteral stenosis, because the increased fluid accumulation will compress the kidney cortex, causing the kidney to enlarge and the kidney cortex to become thinner, and in serious cases, it will cause abnormalities in the structure and function of the kidney. Depending on the child’s age and the condition of the fluid, ureteral drainage or surgical treatment may be chosen to relieve the obstruction and protect the kidney. Therefore, if a child is found to have hydronephrosis during screening, and the hydronephrosis is mild (not more than 10mm), and the child has normal urination, no discomfort and normal growth and development, the child can be reviewed regularly with ultrasound to observe the hydronephrosis and consult a urologist if necessary. If the hydrocele is heavy and causes enlargement of the kidney, dilatation of the renal calyces and thinning of the renal cortex, you should actively consult a urologist to relieve the obstruction symptoms as soon as possible. In general, if hydronephrosis is detected early and the obstruction is removed in time, the residual kidney function can be protected and the prognosis is better. Because hydronephrosis may not show obvious discomfort at the beginning, it is only found during physical examination or in the late stage when the kidney enlarges and a mass appears in the abdomen, or when the patient is seen for recurrent urinary tract infections. Therefore, we suggest that parents should have regular physical examinations even if their children are not in obvious discomfort, and ultrasound is a non-invasive examination method that is recommended at least once every two years.