Active, but not passive cigarette smoking was inversely associated with mammographic density.

<bold>Purpose: </bold>The opposing carcinogenic and antiestrogenic properties of tobacco smoke may explain why epidemiologic studies have not consistently reported positive associations for active smoking and breast cancer risk. A negative relation between mammographic density, a strong breast cancer risk factor, and active smoking would lend support for an antiestrogenic mechanism.<bold>Methods: </bold>We used multivariable linear regression to assess the associations of active smoking and secondhand smoke (SHS) exposure with mammographic density in 799 pre- and early perimenopausal women in the Study of Women's Health Across the Nation (SWAN).<bold>Results: </bold>We observed that current active smoking was associated with 7.2% lower mammographic density, compared to never active smoking and no SHS exposure (p = 0.02). Starting to smoke before 18 years of age and having smoked > or =20 cigarettes/day were also associated with statistically significantly lower percent densities. Among nulliparous women having smoked > or =20 cigarettes/day was associated with 23.8% lower density, compared to having smoked < or = 9 cigarettes/day (p<0.001).<bold>Conclusions: </bold>Our findings support the hypothesis that tobacco smoke exerts an antiestrogenic effect on breast tissue, but counters the known increased risk of breast cancer with smoking prior to first full-term birth. Thus, our data suggest that the antiestrogenic but not the carcinogenic effects of smoking may be reflected by breast density. [ABSTRACT FROM AUTHOR]