Tobacco smoke is highly addictive and has been linked to 20 percent of all deaths in the United States. It contains many cancer-causing chemicals, and almost one third of all cancer deaths are related to tobacco use. Tobacco smoking has generally been considered to have little or no association with breast cancer risk. Newer studies have challenged this conclusion and suggested a connection between smoking and an increased risk of breast cancer, but more investigation is needed to resolve this issue. Passive smoking has been linked with an increased risk of lung cancer and heart disease. Studies have also indicated a possible linkage between passive smoking and breast cancer risk, but settling this concern will require more study. Understanding the potential association of active and passive smoking with breast cancer risk is important, because women have some control over their exposure to tobacco smoke, unlike many other breast cancer risk factors.