Although epidemiological evidence on the role of active cigarette smoking in breast cancer risk has been inconsistent, recent literature supports a modest association between smoking and breast cancer. This association is particularly observed in women who initiate smoking at a younger age, or who smoke for a long time prior to their first pregnancy. Here we provide updated results on cigarette smoking and breast cancer risk in the Canadian National Breast Screening Study (NBSS). The NBSS is a large cohort of 89,835 women, aged 40-59, who were followed for a mean of 22.1 years, resulting in the ascertainment of 6549 incident cases of breast cancer. Cox proportional hazard models were used to estimate hazard ratios (HR) and 95% confidence intervals (CI) for the association of cigarette smoking variables with breast cancer risk. We found breast cancer to be associated with duration (40 years vs 0: HR=1.57; 95%CI=1.29-1.92), intensity (40 cigarettes per day vs 0: HR=1.21; 95%CI=1.04-1.40), cumulative exposure (40 pack-years vs 0: HR=1.19; 95%CI=1.06-1.13), and latency (40 years since initiation vs 0: HR=1.19; 95%CI=1.10-1.53) of cigarette smoking. Number of years smoked prior to first full term pregnancy was associated with higher risk of breast cancer than comparative years smoked post-pregnancy (among parous women, 5 years pre pregnancy vs 0: HR=1.18; 95%CI=1.10-1.26). These results strongly support a role for cigarette smoking in breast cancer etiology and emphasize the importance of timing of this exposure. © 2014 Wiley Periodicals, Inc.