Our study demonstrates that a history of smoking increases a woman’s risk for developing breast cancer.  The difference between this study and many other previous studies is our definition of smoking.  Most previous studies have assessed smoking status only at the time of breast cancer diagnosis. As a result, former smokers may have been misclassified as never smokers and cigarette exposure underestimated. Our study assessed any history of smoking prior to or at the time of the breast cancer diagnosis allowing us to estimate lifetime cigarette exposure. It has been theorized that smoking affects breast cancer development via the estrogen metabolism cycle.  The findings from our study provide new evidence that avoidance of smoking or stopping smoking as soon as possible may reduce breast cancer risk. All women who smoke should be advised to stop smoking and provided behavioral and pharmacologic treatment in accord with clinical practice guidelines.