Medullary sponge kidney is a relatively common and benign condition of the kidney. The diagnosis is usually made by characteristic features seen on imaging tests of the kidney (such as ultrasound, intravenous pyelography (IVP) or CT scan) performed for the evaluation of kidney stones or blood in the urine.

Before urine reaches the bladder, it flows through tiny tubes within the kidney (called tubules) that allow the body to reclaim essential salts and nutrients. Medullary sponge kidney is characterized by small cysts or dilatations (stretching) within these tubules. It is the characteristic appearance of these cysts seen on the imaging studies that permits the diagnosis to be made. The cysts can vary in size from microscopic up to about half an inch, and are thought to be a developmental problem (present at birth). Interestingly, sometimes the cysts are present in only one kidney.

The most common problems relating to medullary sponge kidney are a higher chance of developing kidney stones that can cause back and abdominal pain, blood in the urine, and urinary-tract infection. Urinary bleeding from medullary sponge kidney is usually painless, can be intermittent, and occasionally will result in grossly bloody urine. Despite these abnormalities, patients with medullary sponge kidney do not develop kidney failure and no treatment is needed unless kidney stones or urinary-tract infection are present.