The association between alcohol consumption and breast cancer was investigated in a case-control study involving 1,524 cases and 1,896 controls identified through a nationwide screening program. Ever drinking alcohol was not associated with any substantial increase in risk [odds ratio (OR) = 1.1; 95% confidence interval (Cl) = 1.0-1.3], but there was a significant trend in risk with increasing average weekly intake (P less than .04). Women who had one or fewer drinks daily (83% of all drinkers) did not experience any excess risk compared to nondrinkers, but significant excess risks were observed among those who drank from 1 to 2 (OR = 1.3; 95% Cl = 1.0-1.7) or more than 2 (OR = 1.7; 95% Cl = 1.2-2.4) drinks a day. An increased risk associated with alcohol consumption was evident only for those who drank at younger ages (less than 30 yr), regardless of current consumption. Alcohol effects were adjusted for a variety of factors, including reproductive history, were adjusted for a variety of factors, including reproductive history, socioeconomic indicators, and obesity, but none exerted any appreciable confounding influence. The results support an association between moderate alcohol consumption in early life and subsequent breast cancer risk, although interpretation should be cautious in the absence of dietary information.