Abstract Background: In contrast to the predominance of observational studies, in the Women's Health Initiative (WHI) trial postmenopausal women assigned to estrogen alone had a lower risk of invasive breast cancer than did those assigned placebo (LaCroix JAMA 2011;305:1305-14). Influence of estrogen use on breast cancer mortality has not been previously reported. Methods: We conducted extended follow-up of the WHI randomized, placebo-controlled trial evaluating conjugated equine estrogen (0.625 mg/d) in 10739 postmenopausal women aged 50-79 years with prior hysterectomy, to determine long term effects of estrogen use on non-invasive and invasive breast cancer incidence, invasive breast cancer characteristics and breast cancer mortality. Comprehensive breast cancer risk assessment was conducted at entry. Screening mammography and clinical breast examinations were performed at entry and yearly thereafter. Breast cancers were confirmed by medical record and pathology report review by centrally-trained physician adjudicators blinded to randomization assignment. Analyses were by intention-to-treat. The study is registered with clinical trials CT.govNCT00000611. Findings: After a mean 10.7 years of follow-up, conjugated equine estrogen use for a median of 5.9 years (range <1 to 10 years) was associated with lower invasive breast cancer incidence compared to placebo (151 vs. 199 breast cancers; annualized rates, 0.27% vs. 0.35%; hazard ratio [HR] 0.77; 95% confidence interval [CI], 0.62 to 0.95, P=0.02) with no difference between intervention-phase and post-intervention-phase effects (P=0.76). Sensitivity analyses adjusting for non-adherence yielded a somewhat stronger association (HR 0.68; 95% CI 0.49-0.95). Mammogram screening exams were comparable between randomization groups throughout. In the estrogen alone group fewer women died from breast cancer (6 vs. 16 deaths; annualized rates 0.009% vs. 0.024%; HR 0.37; 95% CI, 0.13 to 0.91, P=0.03) and fewer women died from all causes after a breast cancer diagnosis (30 vs. 50 deaths; annualized rate 0.046% vs. 0.076%; HR 0.62; 95% CI, 0.39 to 0.97, P=0.04).Interpretation: Use of conjugated equine estrogen for a median of 5.9 years in postmenopausal women reduced the incidence of invasive breast cancer and reduced breast cancer mortality. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1031. doi:1538-7445.AM2012-1031