Alcohol consumption is known to be a risk factor for breast cancer in Western countries, but few epidemiologic data have been available in Japan. This population-based prospective cohort study evaluated the associations of alcohol consumption with breast cancer risk in a Japanese population. A total of 19,227 women aged 40-64 years were followed from 1990 to 2003. During 246,703 person-years of follow-up, 241 breast cancer cases were identified. Hazard ratios (HRs) were estimated by the Cox proportional-hazard regression model. After adjustment for potential risk factors of breast cancer and nutritional factors, the HR and 95% confidence interval (CI) for current drinkers was 1.00 (0.74-1.34) compared with never drinkers. According to the amount of alcohol intake per day, a higher amount (≥15.0 g/day) had no significant relation to breast cancer risk (HR = 0.87, 95% CI: 0.40-1.91; P for trend = 0.85). Age upon starting to drink, and the frequency of drinking, were not associated with breast cancer risk. In analysis stratified according to exogenous female hormone use, a higher alcohol intake (≥15.0 g/day) was associated with an increased risk of breast cancer among hormone users (HR = 1.67, 95% CI: 0.17-16.73); however, this was not statistically significant. Stratification according to folate intake with energy adjustment (<219, ≥ 219 μg/day) found that breast cancer risk tended to increase with increasing alcohol consumption among women with a low intake of folate (P for trend = 0.09). Our findings suggest that alcohol consumption has no overall effect on breast cancer risk among Japanese women, whereas nutritional factors such as folate intake may modify the alcohol-breast cancer risk relationship.