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Objective To examine the association between smoking and risk of invasive breast cancer using quantitative measures of lifetime passive and active smoking exposure among postmenopausal women.

Design Prospective cohort study.

Setting 40 clinical centres in the United States.

Participants 79 990 women aged 50–79 enrolled in the Women’s Health Initiative Observational Study during 1993–8.

Main outcome measures Self reported active and passive smoking, pathologically confirmed invasive breast cancer.

Results In total, 3520 incident cases of invasive breast cancer were identified during an average of 10.3 years of follow-up. Compared with women who had never smoked, breast cancer risk was elevated by 9% among former smokers (hazard ratio 1.09 (95% CI 1.02 to 1.17)) and by 16% among current smokers (hazard ratio 1.16 (1.00 to 1.34)). Significantly higher breast cancer risk was observed in active …