Mayo Clinic Health Library


Updated: 08-02-2018


Hydronephrosis is swelling of one or both kidneys. Kidney swelling happens when urine can't drain from a kidney and builds up in the kidney as a result. This can occur from a blockage in the tubes that drain urine from the kidneys (ureters) or from an anatomical defect that doesn't allow urine to drain properly.

Hydronephrosis can happen at any age. Hydronephrosis in children may be diagnosed during infancy or sometimes during a prenatal ultrasound before the baby is born.

Hydronephrosis doesn't always cause symptoms. When they occur, signs and symptoms of hydronephrosis might include:

  • Pain in the side and back (flank pain) that may travel to the lower abdomen or groin
  • Urinary problems, such as pain with urination or feeling an urgent or frequent need to urinate
  • Nausea and vomiting
  • Fever
  • Failure to thrive, in infants


Normally, urine goes from the kidney to the tube that drains the kidney (ureter), to the bladder and then out of the body. But, sometimes urine backs up or remains inside the kidney or in the ureter. That's when hydronephrosis can develop.

Some common causes of hydronephrosis include:

  • Partial blockage in the urinary tract. Urinary tract blockages often form where the kidney meets the ureter, at a point called the ureteropelvic junction. Less commonly, blockages may occur where the ureter meets the bladder at what's called the ureterovesical junction.
  • Vesicoureteral reflux. Vesicoureteral reflux happens when urine flows backward through the ureter from the bladder up into the kidney. Normally, urine flows only one way in the ureter. Urine flowing the wrong way makes it difficult for the kidney to empty properly and causes the kidney to swell.

Less-common causes of hydronephrosis include kidney stones, a tumor in the abdomen or pelvis, and problems with nerves that lead to the bladder.



Tests for diagnosing hydronephrosis may include:

  • A blood test to evaluate kidney function
  • A urine test to check for signs of infection or urinary stones that could cause a blockage
  • An ultrasound imaging exam, during which your doctor can view the kidneys, bladder and other urinary structures to identify potential problems
  • A voiding cystourethrogram, an X-ray exam that uses a special dye to outline the kidneys, ureters, bladder and urethra, capturing images before and during urination

Depending on the results from initial testing, your doctor may recommend additional imaging exams, such as a computerized tomography (CT) scan or magnetic resonance imaging (MRI). Another possibility is a test called a MAG3 scan, a nuclear medicine imaging test that evaluates the function and drainage in the kidney.

You may be referred to a doctor who specializes in conditions affecting the urinary system (urologist).



Treatment for hydronephrosis depends on the underlying cause. Although surgery is sometimes needed, in many cases hydronephrosis resolves on its own.

  • Mild to moderate hydronephrosis. Your doctor may opt for a wait-and-see approach to treatment, as hydronephrosis could resolve on its own. Even so, your doctor may recommend preventive antibiotic therapy to lower the risk of urinary tract infections.
  • Severe hydronephrosis. When hydronephrosis makes it hard for the kidney to function — as can happen in more-severe cases or in cases that involve reflux — surgery may be recommended to eliminate the blockage or correct the reflux.

Left untreated, severe hydronephrosis could lead to permanent kidney damage. Rarely, it can cause kidney failure. In most cases, however, this condition does resolve successfully. In the meantime, because hydronephrosis typically affects only one kidney, the other kidney can do the work for both.