Xue et al evaluated the association of active and passive smoking with the incidence of breast cancer based on data from the Nurses' Health Study, including follow-up of 111 140 women from 1976 to 2006. A total of 8772 incident cases of breast cancer arose during 3 005 863 person-years of follow-up. Ever smokers had a slightly higher incidence of breast cancer than never smokers (hazard ratio [HR], 1.06). The incidence of breast cancer was positively associated with higher quantity of current and past smoking, younger age of initiation, longer duration, and more pack-years of smoking. While premenopausal smoking was associated with a slightly higher incidence of breast cancer (HR, 1.11 for increase of every 20 pack-years) especially smoking before first birth (HR, 1.18 for increase of every 20 pack-years), the direction of the association between postmenopausal smoking and breast cancer was reversed (HR, 0.93 for increase of every 20 pack-years). Passive smoking in childhood or adulthood was not associated with the incidence of breast cancer.