In many studies, cigarette smoking has been associated with a small increase in breast cancer risk. The authors evaluated the relation of smoking to breast cancer risk in two case-control studies carried out from 1982 through 1986. In Canada, 607 women with breast cancer and 1,214 controls matched on decade of age and neighborhood were interviewed at home. In the United States, 1,955 cases of breast cancer and 805 controls with other cancers were interviewed in the hospital. In both studies, breast cancer risk was associated weakly with cigarette smoking overall. The odds ratio for women who had smoked 25 or more cigarettes per day as compared with never smokers was 1.2 (95% confidence interval (CI) 0.9-1.6) in the Canadian study and 1.2 (95% Cl 0.9-1.6) in the US study. In both studies, breast cancer risk was more strongly related to commencement of smoking at a young age. Among women who smoked at least 25 cigarettes per day in the most recent year of smoking, the odds ratios for commencement before age 16 years were 1.7 (95% Cl 1.0-2.9) in the Canadian data and 1.8 (95% Cl 1.0-3.4) in the US data, and the odds ratios for commencement at even younger ages were higher. The associations were not explained by duration of smoking, by the time elapsed since commencement, or by factors associated with cigarette smoking such as alcohol consumption or oral contraceptive use. Our findings raise the hypothesis that exposure to cigarette smoke during adolescence may increase a woman's risk of breast cancer. The hypothesis has biologic plausibility: cigarette smoke contains known carcinogens, and the developing breast is especially susceptible to cancer initiation.