1.Why do the kidneys become hydronephrosis? The most common cause of hydronephrosis is stenosis of the pelvic ureteral junction, in addition to other causes such as lower ureteral stenosis, giant ureter, posterior urethral valves in boys, and neurogenic bladder. Once the prenatal ultrasound reveals hydronephrosis, the fetus should be followed up by both the obstetrician and the pediatric urologist. After the birth of the child, if there is a lump in the back or abdomen, urinary tract infection, or progressive development of hydronephrosis, surgical treatment is required. Children without these symptoms should be followed up every 3 months for 2 years. During the follow-up, if ultrasound examination reveals an increase in the amount of hydronephrosis in the affected kidney, while isotope renogram (a method to check the function of the divided kidney) shows a decrease in kidney function on the side of hydronephrosis, surgery is needed. 2.Will there still be fluid after hydrocele surgery? Surgery for hydronephrosis caused by stenosis of the ureteropelvic junction is performed by removing the stenosis and then connecting the pelvis and ureter so that urine can flow freely from the kidney to the bladder. However, the enlargement of the renal pelvis (that is, where the hydronephrosis is located) still exists after the surgery. As long as the hydronephrosis does not increase in size and the function of the kidney on the hydronephrosis side does not decrease during the follow-up after the surgery, it means that the urine is draining freely and the surgery has been successful. If the hydronephrosis is severe enough to cause almost non-functional unilateral renal function, a nephrostomy should be done first to allow urine to come out of the skin stoma first, and then an isotope renogram should be done 3-6 months later, and if the kidney is still useful then a pelvic dissection pyeloureteral anastomosis should be done.