Diet, inflammation, and oxidative stress may be important in breast carcinogenesis, but evidence on the role of the inflammatory and pro-oxidative potential of dietary patterns is limited. Energy adjusted-Dietary Inflammatory Index (E-DII ) and dietary oxidative balance score (D-OBS) were calculated for 43,563 Sister Study cohort participants who completed a Block 1998 food frequency questionnaire at enrollment in 2003-2009 and satisfied eligibility criteria. D-OBS was validated using measured F -isoprostanes and metabolites. High E-DII score and low D-OBS represent a more pro-inflammatory and pro-oxidant diet, respectively, and associations of quartiles of each index with breast cancer (BC) risk were estimated using multivariable Cox proportional hazards regression. There were 2,619 BCs diagnosed at least 1 year after enrollment (mean follow-up 8.4 years). There was no overall association between E-DII and BC risk, whereas there was a suggestive inverse association for the highest versus lowest quartile of D-OBS (HR 0.92 [95% CI, 0.81-1.03]). The highest quartile of E-DII was associated with risk of triple-negative BC (HR 1.53 [95% CI, 0.99-2.35]). When the two indices were combined, a pro-inflammatory/pro-oxidant diet (highest tertile of E-DII and lowest tertile of D-OBS) was associated with increased risk for all BC (HR 1.13 [95% CI, 1.00-1.27]) and for triple-negative BC (1.72 [95% CI, 1.10-2.70]), compared with an anti-inflammatory/anti-oxidant diet (lowest tertile of E-DII and highest tertile of D-OBS). Diets with increased inflammatory potential and reduced oxidative balance were positively associated with overall and triple-negative BC.