The relationship between smoking and the risk of breast cancer has been unclear for several decades, and whether or not young women should be warned about smoking as a possible cause of breast cancer remains controversial 123. A recent pooled analysis of 53 epidemiologic studies found that the positive relationship with smoking and breast cancer was present only among those consuming alcohol, an established cause of breast cancer 4. The purpose of this report was to examine the effect of smoking on breast cancer risk among non-drinking women in a large population-based cohort of women, many of whom started smoking as teenagers.

We followed 102,098 women, aged 30–50 years, completing a mailed questionnaire at recruitment to the Norwegian–Swedish Cohort Study in 1991/1992, through December 2000. Altogether, 1240 women were diagnosed with incident, invasive breast cancer. We used Cox proportional hazard regression models to estimate the relative risk of breast cancer associated with different measures of smoking initiation, duration, and intensity adjusting for confounding variables. We conducted analyses on the non-drinking study population, and especially among non-drinking women who had smoked for at least 20 years.

We identified 214 breast cancer cases among the 19,288 non-drinking women. Compared with never smokers, women who smoked for at least 20 years, and initiated smoking prior to their first birth (relative risk = 1.75; 95% confidence interval = 1.02-2.99), before menarche (relative risk = 1.66; 95% confidence interval = 0.79-3.49) or before age 15 (relative risk = 2.20; 95% confidence interval = 0.92-5.30), had an increased risk. In contrast, women who had smoked for at least 20 years, but started after their first birth, did not experience an increased breast cancer risk (relative risk = 0.97; 95% confidence interval = 0.44-2.12).

Our results support the notion that non-drinking women who start smoking as teenagers and who continue to smoke for at least 20 years may increase their risk of breast cancer.