This prospective US cohort study, based on the observational arm of the Women’s Health Initiative Study, examined the link between smoking and risk of invasive breast cancer among postmenopausal women.

The study involved 79,990 women aged 50–79 who were in the study during 1993–8. The main outcome measures were self reported active and passive smoking and pathologically confirmed invasive breast cancer.

The following findings were reported:

• 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 (1.16; 1.00 to 1.34).

• Higher breast cancer risk was observed in active smokers with high intensity and duration of smoking, as well as with initiation of smoking in the teenage years.

• The highest breast cancer risk was found among women who had smoked for ≥50 years or more (1.35; 1.03 to1.77) compared with all lifetime non-smokers (1.45; 1.06 to 1.98) compared with lifetime non-smokers with no exposure to passive smoking.

• An increased risk of breast cancer persisted for up to 20 years after smoking cessation.

• Among women who had never smoked, after adjustment for potential confounders, those with the most extensive exposure to passive smoking (≥10 years’ exposure in childhood, ≥20 years’ exposure as an adult at home, and ≥10 years’ exposure as an adult at work) had a 32% excess risk of breast cancer compared with those who had never been exposed to passive smoking (1.32; 1.04 to 1.67). However, there was no significant association in the other groups with lower exposure and no clear dose response to cumulative passive smoking exposure.

The researchers conclude from these findings that “active smoking was associated with an increase in breast cancer risk among postmenopausal women. There was also a suggestion of an association between passive smoking and increased risk of breast cancer.”

According to an accompanying editorial, this “analysis supports the hypothesis that smoking increases the risk of breast cancer, in particular when the habit starts early in life. These data, however, should be placed in the context of the overall evidence, and in particular the results of a pooled analysis of 10 cohort studies and 43 case-control studies, which found no increased risk in either group of studies. In the case of secondhand smoke, the evidence can still be considered suggestive of an association at best.”