Abstract Background: We investigated the associations of postdiagnostic dietary glycemic index (GI), glycemic load (GL), insulin index (II), and insulin load (IL) with breast cancer–specific and all-cause mortality. Methods: Among 8,932 women with stage I–III breast cancer identified in the Nurses' Health Study (NHS; 1980–2010) and NHSII (1991–2011), we prospectively evaluated the associations between postdiagnostic GI, GL, II, and IL, and breast cancer–specific and all-cause mortality. Participants completed a validated food frequency questionnaire every 4 years after diagnosis. Results: During follow-up by 2014 in the NHS and 2015 in the NHSII, 2,523 deaths, including 1,071 from breast cancer, were documented. Higher postdiagnostic GL was associated with higher risk of both breast cancer–specific mortality [HRQ5vsQ1 = 1.33; 95% confidence interval (CI) = 1.09–1.63; Ptrend = 0.008] and all-cause mortality (HRQ5vsQ1 = 1.26; 95% CI = 1.10–1.45; Ptrend = 0.0006). Higher all-cause mortality was also observed with higher postdiagnostic GI (HRQ5vsQ1 = 1.23; 95% CI = 1.08–1.40; Ptrend = 0.001), II (HRQ5vsQ1 = 1.20; 95% CI = 1.04–1.38; Ptrend = 0.005), and IL (HRQ5vsQ1 = 1.23; 95% CI = 1.07–1.42; Ptrend = 0.0003). The associations were not modified by insulin receptor or estrogen receptor status of the tumor, or body mass index. Conclusions: We found that higher dietary GL, reflecting postprandial glucose response, after a breast cancer diagnosis was associated with higher risk of breast cancer–specific mortality. Higher dietary GI, GL, II, and IL after a breast cancer diagnosis were associated with higher risk of death from any cause. Impact: These results suggest that carbohydrate quantity and quality may be important in breast cancer prognosis. See related commentary by McTiernan, p. 252