Whether hematuria in patients with hydronephrosis is serious cannot actually be determined by the amount of blood in the urine. It is important to identify the cause and assess kidney function, because hydronephrosis can lead to serious consequences, mainly kidney failure. If the hydronephrosis persists for a long time, it can compress the renal cortex and cause kidney shrinkage, leading to chronic kidney failure. The dangers of hematuria mainly include: first, it may cause significant bleeding, leading to anemia. Second, this type of hematuria could be caused by certain conditions, such as bleeding caused by a tumor in the urinary system, which is quite serious. Whereas bleeding caused by urinary tract infections or urinary system stones, in terms of the nature of the cause, is not considered very serious. However, significant bleeding can also lead to severe anemia and even shock, which would be more serious.