Kidney stones may form when the normal balance of water, salts, minerals, and other substances found in urine changes. How this balance changes determines the type of kidney stone you have. Most kidney stones are calcium-type-they form when the calcium levels in your urine change.

Factors that change your urine balance include:

• Not drinking enough water. Try to drink enough water to keep your urine clear (about 8 to 10 glasses of water a day). When you don't drink enough water, the salts, minerals, and other substances in the urine can stick together and form a stone. This is the most common cause of kidney stones.
• Medical conditions. Many medical conditions can affect the normal balance and cause stones to form. Gout is one example. Also, people who have inflammatory bowel disease or who have had surgery on their intestines may not absorb fat from their intestines in a normal way. This changes the way the intestines process calcium and other minerals, and it may lead to kidney stones.

More commonly, kidney stones can run in families, as they often occur in family members over several generations.

In rare cases, a person forms kidney stones because the parathyroid glands produce too much of a hormone, which leads to higher calcium levels and possibly calcium kidney stones.