To provide additional data on the smoking-breast cancer association, a case-control study of 456 cases of breast cancer and 1693 matched controls was conducted among participants in a cancer screening program. The adjusted risk of breast cancer for current smokers was 1.38 (95% confidence interval, 1.01 to 1.90). Analysis of smoking habits restricted to premenopausal women revealed a risk estimate of 2.33 (confidence interval, 1.10 to 4.96) among current smokers and increasing linear trends in risk for number of cigarettes smoked per day and for number of years of smoking. Although smokers had an earlier natural menopause than nonsmokers, there was no evidence of a protective effect of early menopause after adjustment for other factors. These findings suggest that smoking may increase the incidence of breast cancer, especially in premenopausal women.