Swelling in one or both kidneys due to a backup of urine.

Hydronephrosis is caused by a blockage in the tube that connects the kidney to the bladder. This tube is called the ureter. Hydronephrosis also can happen when urine flows the wrong way through the ureter. Other causes include kidney stones, tumors and problems with nerves to the bladder.

There might be no symptoms. Symptoms can include pain in the side, back or groin and trouble urinating.

This condition sometimes clears on its own. Treatment might include antibiotics to prevent urinary tract infections or surgery to clear a blockage.