The exact causes of breast cancer are not known. However, studies show that the risk of breast cancer increases as a woman gets older. This disease is very uncommon in women under the age of 35. Most breast cancers occur in women over the age of 50, and the risk is especially high for women over age 60. Also, breast cancer occurs more often in white women than African American or Asian women.

Research has shown that the following conditions increase a woman's chances of getting breast cancer:

• Personal history of breast cancer. Women who have had breast cancer face an increased risk of getting breast cancer in their other breast.

• Family history. A woman's risk for developing breast cancer increases if her mother, sister, or daughter had breast cancer, especially at a young age.

• Certain breast changes. Having a diagnosis of atypical hyperplasia or lobular carcinoma in situ (LCIS) may increase a woman's risk for developing cancer.

• Genetic alterations. Changes in certain genes increase the risk of breast cancer. In families in which many women have had the disease, gene testing can sometimes show the presence of specific genetic changes that increase the risk of breast cancer. Doctors may suggest ways to try to delay or prevent breast cancer, or to improve the detection of this disease in women who have these changes in their genes.

Other factors associated with an increased risk for breast cancer include:

• Estrogen. Evidence suggests that the longer a woman is exposed to estrogen (estrogen made by the body, taken as a drug, or delivered by a patch), the more likely she is to develop breast cancer. For example, the risk is somewhat increased among women who began menstruation at an early age (before age 12), experienced menopause late (after age 55), never had children, or took hormone replacement therapy for long periods of time. Each of these factors increases the amount of time a woman's body is exposed to estrogen.

• Late childbearing. Women who have their first child late (after about age 30) have a greater chance of developing breast cancer than women who have a child at a younger age.

• Breast density. Breasts that have a high proportion of lobular and ductal tissue appear dense on mammograms. Breast cancers nearly always develop in lobular or ductal tissue (not fatty tissue). That's why cancer is more likely to occur in breasts that have a lot of lobular and ductal tissue (that is, dense tissue) than in breasts with a lot of fatty tissue. In addition, when breasts are dense, it is more difficult for doctors to see abnormal areas on a mammogram.

• Radiation therapy. Women whose breasts were exposed to radiation during radiation therapy before age 30, especially those who were treated with radiation for Hodgkin's disease, are at an increased risk for developing breast cancer. Studies show that the younger a woman was when she received her treatment, the higher her risk for developing breast cancer later in life.

• Alcohol. Some studies suggest a slightly higher risk of breast cancer among women who drink alcohol.

Most women who develop breast cancer have none of the risk factors listed above, other than the risk that comes with growing older. Scientists are conducting research into the causes of breast cancer to learn more about risk factors and ways of preventing this disease.