Hydronephrosis is swelling inside the part of the kidney called the pelvis. Hydronephrosis can occur on one side of the body (unilateral) or both sides (bilateral). Hydronephrosis is not a disease in and of itself. It is caused by blockage of urine flow out of the kidney. The blockage can be in the tube that connects the kidney to the bladder (ureter), inside the bladder or inside the urethra (the tube that drains urine outside the body. Causes include an enlarged prostate in men, kidney stones, scarring, pregnancy and tumors