Whether the condition of hydronephrosis is serious depends on various factors such as the duration of hydronephrosis, the cause, the extent of fluid accumulation, and more. For example, some patients may have mild hydronephrosis in only one kidney and it may have developed recently; generally, this condition is not very serious. Some patients might have urinary obstructions caused by urinary system stones, and the issue could be resolved using extracorporeal shock wave lithotripsy or surgical stone removal, making the condition not serious. However, in some cases, the cause could be bladder cancer or a tumor near the urinary system compressing the ureter, and if the tumor is difficult to remove, the hydronephrosis might be hard to manage. Long-term fluid accumulation might lead to kidney failure, and these scenarios are considered quite serious.