1569 Background: Associations between dietary protein intake and breast cancer are unclear, in part due to limitations of dietary self-report. Women’s Health Initiative (WHI) investigators compared the accuracy of food frequency questionnaire (FFQ) data on energy and protein intake with objective measures of dietary intake using biomarkers (doubly labeled water for energy and urinary nitrogen for protein [n=544]). Subsequently, regression equations incorporating participant characteristics were developed acknowledging differential reporting dietary data errors based on participant characteristics (Neuhouser Am J Epidemiol). FFQ findings were then used to determine biomarker- adjusted animal vs vegetable protein ratios. Methods: We examined associations of energy and protein intake with breast cancer incidence and mortality in Women’s Health Initiative (WHI) participants 50-79 years of age at entry between1993-1998, with breast cancers verified by medical record review and survival enhanced by serial National Death Index (NDI) searches through 2016. Associations between sources of protein intake (animal versus vegetable) quintiles and breast cancer incidence and mortality were estimated using multivariable Cox proportional hazards regression. Results: With 100,024 eligible participants, after 14 years follow-up, women with higher total protein intake had greater body mass index, were more likely White, menopausal hormone therapy users with higher total energy intake and fat intake. With 6,340 incident breast cancers, 764 deaths from breast cancer and 2,059 deaths after breast cancer, higher vegetable protein intake was associated with significantly lower breast cancer incidence (P for linear trend = 0.01) while higher animal protein intake was associated with significantly higher breast cancer incidence (P for linear trend = 0.03). Higher vegetable protein intake was also associated with significantly lower risk of death after breast cancer (P <0.001) but not with lower risk of deaths from breast cancer (breast cancer followed by death attributed to breast cancer). Animal protein intake was not associated with deaths from breast cancer or deaths after breast cancer. Conclusions: Based on findings from biomarker-calibrated determination of protein intake by source, higher vegetable protein intake was associated with significantly lower risk of breast cancer incidence and of death after breast cancer while higher animal protein intake was associated with significantly higher risk of breast cancer incidence, but not mortality.