This is a multi-center prospective non-randomized single arm, open label trial in postmenopausal HR positive early breast cancer patients who experience grade 2-3 arthralgia-myalgia while on anastrozole, resulting in the discontinuation of anastrozole. After a 2-3 week period without any aromatase inhibitor treatment, eligible patients will initiate letrozole treatment at a dose of 2.5mg per day for a duration of 24 weeks. If a patient has breast cancer recurrence or is intolerant to letrozole during the 24 week period, the drug will be discontinued.