Smoking, menopause, and breast cancer.

As part of a multiphasic health examination, breast cancer incidence was studied in a cohort of 84,172 women who reported their smoking habits between 1964 and 1972. After adjustment for race, education, marital status, body mass, parity, age of menarche, and alcohol consumption, women currently smoking at examination had a slight increase in the relative risk of breast cancer; relative risk for light smokers was 0.95 (95% confidence interval--0.80, 1.11); for moderate smokers, 1.22 (1.05, 1.43); and for heavy smokers, 1.19 (0.88, 1.60) compared with non-smokers. Consistent with the hypothesis that estrogen levels mediate a small protective effect of smoking on the risk of breast cancer, we found that moderate and heavy smoking were associated with a 1.29-year and a 1.48-year reduction in the age of menopause. A 1-year reduction in age of menopause was associated with a 0.97 reduction in the risk of breast cancer. However, there was no indication that the overall effect of smoking on breast cancer was protective.