The association of smoking with outcomes following breast cancer prognosis is not well understood. In a cohort study called Life After Cancer Epidemiology (LACE), 2,265 women diagnosed with breast cancer were followed for a median of 12 years. We used multivariable proportional-hazards models to determine whether smoking, assessed approximately two years post-diagnosis, was associated with risk of death among these women. We also undertook a systematic review of all cohort studies to date that have examined the association between smoking and breast cancer mortality. Compared with never smokers, women who were current smokers had a twofold higher rate of dying from breast cancer [hazard ratio (HR) = 2.01, 95 % confidence interval (CI) 1.27-3.18] and an approximately fourfold higher rate of dying from competing (non-breast cancer) causes (HR = 3.84, 95 % CI 2.50-5.89). Among seven studies that met the inclusion criteria in the systematic review, three studies and our own reported significantly increased risk of breast cancer death with current smoking. We found little evidence of an association between former smoking and breast cancer mortality (HR = 1.24, 95 % CI 0.94-1.64). Consistent with findings from our prospective observational study, the systematic review of seven additional studies indicates positive association of current smoking with breast cancer mortality, but weak association with former smoking. Women who smoke following breast cancer diagnosis and treatment are at higher risk of death both from breast cancer and other causes.