Purpose Greater protein intake has been associated with better breast cancer survival in several prospective studies, including among 1,982 women in the Nurses' Health Study. We proposed to extend this previous finding. We hypothesized that protein, essential amino acid, branched-chain amino acid, and leucine intakes are associated with improved survival and that these associations are stronger in tumors expressing insulin receptor (IR). Patients and Methods We included 6,348 women diagnosed with stage I to III breast cancer between 1976 and 2004. There were 1,046 distant recurrences. Relative risks (RRs) and 95% CIs were calculated according to quintiles of updated postdiagnostic diet using adjusted Cox proportional hazards models based on follow-up until 2010. Results There was an inverse association between energy-adjusted protein intake and recurrence. Multivariable RRs for increasing quintiles of intake compared with the lowest were 0.95 (95% CI, 0.79 to 1.15), 0.92 (95% CI, 0.76 to 1.11), 0.75 (95% CI, 0.61 to 0.91), and 0.84 (95% CI, 0.69 to 1.03; trend P = .02). For animal protein intake, the RRs were 0.88 (95% CI, 0.73 to 1.06), 0.85 (95% CI, 0.70 to 1.02), 0.75 (95% CI, 0.62 to 0.92), and 0.78 (95% CI, 0.63 to 0.95; trend P = .003). Neither essential amino acids, branched-chain amino acids, nor any individual amino acid stood out as being the source of the association. The association also did not differ by IR status. There was no clear association with any protein-containing foods. Conclusion We found a modest survival advantage with higher intake of protein, regardless of IR status. There was no clear mechanism for this association, although it is consistent with prior studies. Our data suggest that there is likely no advantage for women with a history of breast cancer in restricting protein intake or protein-containing foods.