hello there. hydronephrosis is caused when there is a physical obstruction to the urine flow. urine is formed in the kidneys and travel to the bladder via ureters, and from bladder it comes out through urethra. the obstruction could lie anywhere from just outside the kidney in the ureters to the urethra. the obstruction could be either because of some congenital defect or some acquired cause. for example, urinary stone is also a obstruction, if not treated it may lead to hydronephrosis. anything from the surrounding tissue may press on ureters and cause compression and may therefore lead to hydronephrosis. the treatment includes removal of the cause, therefore surgery may be required for certain causes and not for certain causes. therefore, its necessary to find out what is causing it, visit a surgeon and he will be able to help you out further. hope this helps. regards