The effects of cigarette smoking and alcohol consumption on breast cancer risk were investigated in 276 primary, histologically confirmed breast cancer patients and 1,519 community-based comparison subjects identified in 1977 and 1978 in North Carolina. Data on both behaviors and other pertinent personal and medical characteristics were obtained by interview. Analytic methods included stratification and logistic regression. Among current cigarette smokers of 1-20 cigarettes per day and over 20 per day, the odds ratios (ORs) adjusted for age, race, alcohol consumption, estrogen use, and oral contraceptive use for breast cancer were 0.75 [95% confidence interval (CI) 0.52-1.09] and 0.57 (95% CI 0.30-1.08), respectively. A decrease in risk was not seen in former smokers. With respect to alcohol consumption, the adjusted OR for those having one drink or more per week compared to those having less than one was 1.45 (95% CI 0.99-2.12). If the comparison was ever versus never drinkers, the adjusted OR was 1.47 (95% CI 1.10-1.97); for current drinkers versus nondrinkers, the adjusted OR was 1.89 (95% CI 1.40-2.56). The ORs were adjusted for age, race, cigarette smoking, estrogen use, an oral contraceptive use. These data supports those reports showing an inverse association of cigarette smoking and a positive association of alcohol consumption with breast cancer risk.