In 1980, 89,538 U.S. women 34 to 59 years of age, with no history of cancer, completed an independently validated dietary questionnaire that included the use of beer, wine, and liquor. During the ensuring four years, 601 cases of breast cancer were diagnosed among cohort members. Among the women consuming 5 to 14 g of alcohol daily (about three to nine drinks per week), the age-adjusted relative risk of breast cancer was 1.3 (95 percent confidence limits, 1.1 and 1.7). Consumption of 15 g of alcohol or more per day was associated with a relative risk of 1.6 (95 percent confidence limits, 1.3 and 2.0; Mantel extension chi for linear trend = +4.2; P less than 0.0001). Adjustment for known breast cancer risk factors and a variety of nutritional variables did not materially alter this relation. Significant associations were observed for beer and liquor when considered separately. Among women without risk factors for breast cancer who were under 55 years of age, the relative risk associated with consumption of 15 g of alcohol or more per day was 2.5 (95 percent confidence limits, 1.5 and 4.2). These prospective data derived from measurements of alcohol intake recorded before the diagnosis of breast cancer confirm the findings of several previous case-control studies. Viewed collectively, they suggest that alcohol intake may contribute to the risk of breast cancer.