In the United States, hundreds of thousands of premature deaths each year are linked to smoking. Smoking remains one of the most important public health issues. Smoking affects hormone concentrations, and many studies have found a link between smoking and the risk of breast cancer perhaps related to the duration of smoking. Smoking increases overall mortality, but a link between smoking and breast cancer prognosis has not been established. Researchers analyzed data from the Breast Cancer and Women’s Longevity Study, a prospective population-based observational study conducted in Wisconsin, New Hampshire and Massachusetts, to evaluate the association between smoking status before and after breast cancer prognosis, as well as mortality. A total of 20,691 subjects, aged 20-79 years, were diagnosed with breast cancer between 1998 and 2008. Four hundred and fifty-two of the subjects were revisited a median of six years after their diagnosis. Risk ratios for breast cancer death were calculated based on smoking status. The median time was 12 years, during which a total of 6778 women died, including 2894 of those who died from breast cancer. Compared to women who never smoked, women who were active smokers one year before breast cancer diagnosis were more likely to die from breast cancer (HR, 1.25; 95% CI, 1.13-1.37), respiratory cancers (HR, 14.48; 95% CI, 9.89-21.21) and other respiratory diseases (HR, 6.02; 95% CI, 4.55- 7.97) and cardiovascular disease (HR, 2.08; 95% CI, 1.80- 2.41). Women who continued to smoke after breast cancer diagnosis were more likely to die from breast cancer relative to never-smokers (HR, 1.72; 95% CI, 1.13- 2.60). Those who quit smoking after diagnosis were less likely to die from breast cancer (HR, 0.67; 95% CI, 0.38 – 1.19) and respiratory cancer (HR, 0.39; 95% CI, 0.16 – 0.95) than women who continued to smoke after diagnosis. This study suggests that women who smoked before and after breast cancer diagnosis had a higher mortality rate, not only due to breast cancer, but also possibly from other diseases.