Background: The aim of this study was to analyse the impact of alcohol intake and drinking pattern on the risk of breast cancer.

Methods: A total of 17 647 nurses were followed from 1993 until the end of 2001. At baseline participants completed a questionnaire on alcohol intake and other lifestyle-related factors. Data were analysed using Cox's proportional hazard model.

Results: During follow-up 457 women were diagnosed with breast cancer. The relative risk of breast cancer was 2.30 [Confidence interval (CI): 1.56-3.39] for alcohol intake of 22-27 drinks per week, compared to 1-3 drinks per week. Among alcohol consumers, weekly alcohol intake increased the risk of breast cancer with 2% for each additional drink consumed. Weekend consumption increased the risk with 4% for each additional drink consumed friday through sunday. Binge drinking of 4-5 drinks the latest weekday increased risk with 55%, compared with consumption of one drink. A possible threshold in risk estimates was found for consumption above 27 drinks per week.

Conclusions: For alcohol consumption above the intake most frequently reported, the risk of breast cancer is increased. The risk is minor for moderate levels but increases for each additional drink consumed during the week. Weekend consumption and binge drinking imply an additional increase in breast cancer risk.