In the United States, breast cancer is the third most common cause of cancer death (after lung cancer and colon cancer). In 2007, breast cancer is expected to cause 40,910 deaths (7% of cancer deaths; almost 2% of all deaths) in the U.S.^[2] Among women in the U.S., breast cancer is the most common cancer and the second-most common cause of cancer death (after lung cancer).^[2] Women in the U.S. have a 1 in 8 lifetime chance of developing invasive breast cancer and a 1 in 33 chance of breast cancer causing their death.^[3] In the U.S., both incidence and death rates for breast cancer have been declining in the last few years.^[4] Nevertheless, a U.S. study conducted in 2005 by the Society for Women's Health Research indicated that breast cancer remains the most feared disease,^[5] even though heart disease is a much more common cause of death among women.^[6]

The number of cases worldwide has significantly increased since the 1970s, a phenomenon partly blamed on modern lifestyles in the Western world.^[7][8] Because the breast is composed of identical tissues in males and females, breast cancer also occurs in males, though it is less common.

Breast cancer is a cancer of the glandular breast tissue.

Worldwide, breast cancer is the fifth most common cause of cancer death (after lung cancer, stomach cancer, liver cancer, and colon cancer).^[1] In 2005, breast cancer caused 502,000 deaths (7% of cancer deaths; almost 1% of all deaths) worldwide.^[1] Among women worldwide, breast cancer is the most common cause of cancer death.^[1]