Background: Although an association between alcohol consumption and risk of breast cancer has been observed in many studies, questions of major importance remain, including the nature of the dose–response relationship and the effects of drinking at various periods in life. Purpose: Our goal was to address the issues listed above with a large case–control study. Methods: We conducted a population-based case–control study in Maine, Massachusetts (excluding the four counties that include metropolitan Boston), New Hampshire, and Wisconsin. Case patients were eligible if their diagnosis of invasive breast cancer was first reported to one of the four statewide cancer registries during the period of through 1991. During the accrual period, 11 879 potentially eligible case patients and 16 217 control subjects were identified. After excluding ineligible women from the study, telephone interviews were obtained from 6888 case patients and 9424 control subjects. Complete data for recent alcohol consumption, and thus final eligibility for study participation, were determined for 6662 case patients and 9163 control subjects. The average age at time of interview was 58.7 years. The questions on alcohol use addressed average consumption during five periods of the subjects' lives: ages 16–19, 20–29, 30–39, 40–59, and 60–74 years. Similar responses from 211 control subjects upon reinterview 6–12 months later were taken to be indicative of the reliability of the questionnaire used in this study. Results: Lifetime average alcohol consumption (measured as the average grams per day consumed from age 16 to the recent past) and recent alcohol consumption (average grams per day consumed in the previous age interval) were associated with risk of developing breast cancer. The multivariate relative risk of breast cancer, in those who drink compared with abstainers, associated with average lifetime consumption of 12–18 g/day of alcohol (about one drink) was 1.39 (95% confidence interval [CI] = 1.16–1.67), of 19–32 g/day (about two drinks) was 1.69 (95% CI = 1.36–2.10), of 33–45 g/day (about three drinks) was 2.30 (95% CI = 1.51–3.51), and of greater than or equal to 46 g/day (four or more drinks) was 1.75 (95% CI = 1.16–2.64) ( P for trend <.0001). The multivariate relative risk per 13 g/day (about one drink) of alcohol consumed before 30 years of age was 1.09 (95% CI = 0.95–1.24), whereas the relative risk associated with recent consumption of 13 g/day was 1.21 (95% CI = 1.09–1.34). Conclusions: In these data, alcohol consumption was clearly related to breast cancer risk. Risk appeared to increase even at moderate levels of consumption. For women of all ages combined, consumption before 30 years of age was not an important determinant of risk. [J Natl Cancer Inst 87:923–929, 1995]