The relationship between alcoholic beverage drinking and the risk of breast cancer was considered using data from a case-control study of breast cancer conducted between 1990 and 1995 in the Swiss Canton of Vaud on 230 incident cases of breast cancer below age 75 years, linked with the Vaud Cancer Registry, and 507 controls admitted to the same network of hospitals for a wide spectrum of acute, non-neoplastic, non-hormone-related conditions. Overall, 70.4% of cases versus 57.4% of controls consumed alcohol, corresponding to a multivariate odds ratio (OR) of 1.5 (95% confidence interval (CI): 1.1-2.2). The ORs were 1.3 for < 1 drink per day, 1.8 for 1 to 2, 1.5 for 2 to 4, and 2.7 for > 4 drinks per day, and the trend in risk with dose was significant. The association was consistent for wine (OR = 2.0), beer (OR = 2.6) and spirits (OR = 2.0) and was apparently stronger in premenopausal women, whereas no noticeable interaction was observed with any of the hormonal or reproductive risk factors for breast cancer. The alcohol-related risk was unrelated to duration; the OR was 1.8 for women who started drinking below the age of 30 years and 1.4 for those starting at the age of > or = 30 years. Thus, the present study confirms that alcohol is a correlate of breast cancer risk in this European population, where alcohol drinking among women is common and relatively high. Assuming that this association reflects causality, in terms of attributable risk, alcohol could explain 25% (8-42%) of breast cancer cases.