It has been suggested that cigarette smoking may reduce the incidence of breast cancer, perhaps by as much as 20 per cent. To evaluate the relation between breast-cancer risk and smoking, we studied 2160 women with breast cancer and 717 controls who had been admitted to the hospital for cancer of the ovary, cancer of the colon or rectum, malignant melanoma, or lymphoreticular cancers. As compared with women who had never smoked, the estimated relative risk of breast cancer was 1.1 for current smokers of any amount (95 per cent confidence interval, 0.9 to 1.3), and 1.0 (0.8 to 1.3) for heavy smokers (15 or more cigarettes per day). Allowance for all identified potential confounding factors did not materially alter the results. There was no indication that age at commencement of smoking was related to the risk, nor was there evidence of an effect of smoking within the categories of age at first pregnancy or age at menopause. The data provide evidence against the hypothesis that smoking may reduce the incidence of breast cancer by 20 per cent.