BACKGROUND: We aimed to estimate the effect of alcohol consumption on breast cancer risk and to test whether overweight and obesity modifies this association. METHODS: We included in the analysis 45,233 women enrolled in the Swedish Women's Lifestyle and Health study between 1991 and 1992. Participants were followed for occurrence of breast cancer and death until December 2009. Poisson regression models were used, and analyses were done for overall breast cancer and for estrogen receptor positive or negative (ER+, ER-) and progesterone receptor positive and negative (PR+, PR-) tumors separately. RESULTS: A total of 1,385 breast cancer cases were ascertained during the follow-up period. Overall, we found no statistically significant association between alcohol intake and breast cancer risk after adjustment for confounding, with an estimated relative risk (RR) of 1.01 (95 % CI: 0.98-1.04) for an increment in alcohol consumption of 5 g/day. A statistically significant elevated breast cancer risk associated with higher alcohol consumption was found only among women with BMI =25 (RR 1.03, 95 % CI 1.0-1.05 per 5 g/day increase). CONCLUSION: An increase in breast cancer risk with higher alcohol consumption was found for breast cancers in women with a BMI =25 kg/m(2).