Letrozole in Treating Breast Cancer in Postmenopausal Women With Stage I, II, or III Breast Cancer Previously Treated With Tamoxifen (GIM4)

INCLUSION CRITERIA * Signed informed consent prior to beginning protocol specific procedures. * Histologically proven breast cancer at the first diagnosis with tumor stage I-II-III. Patients with histologically documented (microscopic) infiltration of the skin (pT4) will also be eligible. * Axillary Nodal status allowed: Nx, pNo, pN1, pN2, pN3. * Postmenopausal status defined by one of the following: * Age \> 55 years with cessation of menses * Age \< 55 years but not spontaneous menses for at least 1 year * Age \< 55 years and spontaneous menses within the past 1 year, but currently amenorrheic (e.g. spontaneous, or secondary to hysterectomy), AND with postmenopausal gonadotrophin levels (luteinizing hormone and follicle stimulating hormone levels \>40 IU/L) or postmenopausal estradiol levels (\<5 ng/dL) or according to the definition of "postmenopausal range" for the laboratory involved. * Bilateral oophorectomy * Adjuvant TAM received for at least 2 years and not more than 3 years and 3 months. Patients treated with adjuvant chemotherapy, are required to have begun receiving TAM within 3 months after the completion of chemotherapy. * Definitive surgical treatment must be either mastectomy or breast conserving surgery, with axillary lymph node dissection or sentinel node biopsy for operable breast cancer. * ECOG/WHO performance Status 0-1. Patients must be accessible for treatment and follow-up. * Concomitant treatment with biphosphonates are allowed and should be recorded during the trial. EXCLUSION CRITERIA * Male patients. * Any locally advanced (T4) or inflammatory breast cancer. However, patients with microscopic infiltration of the skin (pT4) will be eligible. * Patients with distant metastases. Any suspicious manifestation requires appropriate investigation to exclude metastases. * Histology other than adenocarcinoma. * Patients with previous or concomitant (not breast cancer) malignancy within the past 5 years EXCEPT adequately treated basal or squamous cell carcinoma of the skin or in situ carcinoma of the cervix. Patients who have had a previous other malignancy must have been disease free for 5 years. * Patients with other non-malignant systemic diseases (cardiovascular, renal, hepatic, lung embolism, etc.) which would prevent prolonged follow-up. * Use of hormone Replacement Therapy within four weeks before randomization. * Concurrent treatment with other experimental drugs. Participation in another clinical trial with any investigational regimen within 30 days prior to study entry. * Concurrent treatment with any other anti-cancer therapy.