Patients who complete ≥ 4.5 years of letrozole (arm I) and who did not experience recurrent disease or new primary breast cancer, including ductal carcinoma in situ, may participate in the double-blind, placebo-controlled, re-randomization portion of the study. Patients are stratified according to lymph node status at enrollment (negative vs positive vs unknown), prior adjuvant chemotherapy (yes vs no), and interval between last dose of letrozole and re-randomization (<6 months vs 6 months to 2 years). Common genetic single nucleotide polymorphisms for genes encoding proteins involved in pharmacokinetic and/or pharmacodynamic pathways for letrozole are analyzed in order to determine if these single nucleotide polymorphisms contribute to individual variation in toxicity and efficacy of letrozole therapy.