A kidney stone is a solid mass made up of tiny crystals. One or more stones can be in the kidney or ureter at the same time.

See also: Cystinuria

Kidney stones can form when urine contains too much of certain substances. These substances can create small crystals that become stones.

The biggest risk factor for kidney stones is dehydration.

Kidney stones may not produce symptoms until they begin to move down the tubes (ureters) through which urine empties into the bladder. When this happens, the stones can block the flow of urine out of the kidneys. This causes swelling of the kidney or kidneys, causing pain. The pain is usually severe.

Kidney stones are common. A person who has had kidney stones often gets them again in the future. Kidney stones often occur in premature infants.

Some types of stones tend to run in families. Certain kinds of stones can occur with bowel disease, ileal bypass for obesity, or renal tubule defects.

There are different types of kidney stones. The exact cause depends on the type of stone.

• Calcium stones are most common. They occur more often in men than in women, and usually appear between ages 20 - 30. They are likely to come back. Calcium can combine with other substances, such as oxalate (the most common substance), phosphate, or carbonate to form the stone. Oxalate is present in certain foods. Diseases of the small intestine increase the risk of forming calcium oxalate stones.
• Cystine stones can form in people who have cystinuria. This disorder runs in families and affects both men and women.
• Struvite stones are mostly found in women who have a urinary tract infection. These stones can grow very large and can block the kidney, ureter, or bladder.
• Uric acid stones are more common in men than in women. They can occur with gout or chemotherapy.

Other substances also can form stones.