Study summary: Women who are diagnosed with breast cancer in one breast are at increased risk of subsequently developing a second primary breast cancer in the opposite breast. The current study examined the impact of alcohol consumption and smoking on asynchronous contralateral breast cancer in the Womens Environmental Cancer and Radiation Epidemiology Study (1985-2001). The women studied, who were under 55 years of age upon first breast cancer diagnosis, were identified from five population-based cancer registries in the U.S. and Denmark. 708 women with asynchronous contralateral breast cancer (the cases) were selected and compared to 1,399 women with breast cancer in one breast (the controls). Cases and controls were matched with respect to birth year, diagnosis year, registry region, race and whether they had received radiation treatment. Information concerning breast cancer risk factors was collected by means of telephone interview. Ever regular drinking was found to be associated with an increased risk of asynchronous contralateral breast cancer (rate ratio = 1.3, 95% confidence interval: 1.0-1.6) and the risk was found to increase over time. Smoking was found not to be related to subsequent contralateral breast cancer.