Background: Alcohol consumption is a well-established risk factor for breast cancer, but the evidence is mostly from developed countries. Brazil is going through a rapid demographic expansion, and studies of this relationship are also needed in such unexplored settings. Methods: We assessed the relationship between alcohol consumption and breast cancer risk among 1,506 Brazilian women (406 cases and 1,100 controls). Regression models were used to calculate odds ratios (OR) and 95% confidence intervals (CI). All statistical tests were two-tailed. Results: The mean age of the 1,506 women was 42.0 (standard deviation, ±15.0) years. There was a significant association between breast cancer and age, body mass index, age at menarche, menstrual flow and menstrual cycle. Multivariate analysis showed an increased risk of invasive breast cancer in regular alcohol consumers (<50 years old: OR 4.7; 95% CI 1.4–16.2; ≥50 years old: OR 3.9; 95% CI 1.2–13.4) compared with abstainers or occasional drinkers. Women with a regular alcohol intake for 10 years or more who were less than 50 years old had a threefold higher risk of developing breast cancer (OR 3.0; 95% CI 1.2–7.6). Conclusion: Regular alcohol consumption increases the risk of breast cancer mainly among women less than 50 years old.