Background:The relationship between cigarette smoking and breast cancer risk has been inconsistent, potentially due to modification by other factors or confounding.Methods:We examined smoking and breast cancer risk in a prospective cohort of 186 150 female AARP (formerly American Association of Retired Persons) members, ages 50-71 years, who joined the study in 1995-96 by responding to a questionnaire. Through 2006, 7481 breast cancers were diagnosed. Multivariable-adjusted hazard ratios (HRs) were estimated, overall and stratified by breast cancer risk factors, using Cox proportional hazards regression. Multiplicative interactions were evaluated using the likelihood ratio test.Results:Increased breast cancer risk was associated with current (HR 1.19, 95% confidence interval (CI) 1.10-1.28) and former (HR 1.07, CI 1.01-1.13) smoking. The current smoking association was stronger among women without (HR 1.24, CI 1.15-1.35) as compared to those with a family history of breast cancer (HR 0.94, CI 0.78-1.13) (P-interaction=0.03). The current smoking association was also stronger among those with later (15 years: HR 1.52, CI 1.20-1.94) as compared with earlier (12 years: HR 1.14, CI 1.03-1.27; 13-14 years: HR 1.18, CI 1.05-1.32) ages at menarche (P-interaction=0.03).Conclusions:Risk was elevated in smokers, particularly in those without a family history or late menarche. Research into smoking's effects on the genome and breast development may clarify these relationships.British Journal of Cancer advance online publication, 18 March 2014; doi:10.1038/bjc.2014.132 www.bjcancer.com.