Background: Observational studies have commonly linked higher alcohol consumption with a modest increase in invasive breast cancer risk, but cohort studies have not examined alcohol intake in relation to ductal carcinoma in situ (DCIS). Methods: The association between adulthood alcohol consumption assessed at baseline and subsequent DCIS risk was examined in a cohort of postmenopausal women participating in theWomen's Health Initiative clinical trials, in which mammography was protocol-mandated. Alcohol intake was assessed by a semi-quantitative food-frequency questionnaire. Reported DCIS cases were verified by central pathology report review. Cox proportional hazards models were used to estimate hazard ratios and 95% confidence intervals. Results: The cohort consisted of 63,822 women with information on alcohol intake, among whom 489 cases of DCIS were ascertained after a median follow-up of 8.0 years. For the primary analysis, invasive breast cancer was treated as a competing risk, and follow-up time was censored at the date of diagnosis of invasive breast cancer. After adjustment for covariates, the hazard ratio for DCIS among women who consumed 14 or more servings of alcohol per week, relative to nondrinkers, was 0.87 (95% confidence interval, 0.50-1.51). In addition, alcohol intake was not associated with risk of either high-grade or low-/moderate-grade DCIS. Conclusions: In this large cohort study of postmenopausal women, alcohol consumption was not associated with risk of DCIS. Impact: If other studies confirm our findings, this would suggest that alcohol may have an effect later in the carcinogenic process. ©2010 AACR.