The influence of alcohol consumption on breast cancer risk was evaluated in a population-based case-control study, including 1,486 cases diagnosed over a one-year (1983-84) period in Denmark. Cases were identified from the files of the nationwide clinical trial of the Danish Breast Cancer Cooperative Group and the Danish Cancer Registry. The control group was an age-stratified random sample of 1,336 women from the general population. Data on risk factors were collected by self-administered questionnaires. The association of alcohol consumption with breast cancer risk varied with age and dietary fat intake. Among women aged 50-59 years, with a fat intake in the lowest quartile, the risk of breast cancer increased with increasing consumption of alcohol. A consumption of 24 g or more per day was associated with an 18-fold increased risk compared with abstainers. For women in other age groups, alcohol consumption had no significant association with breast cancer risk.