Inclusion Criteria:

• Pathological diagnoses of breast cancer.

• Postmenopausal women, defined as:

  • Bilateral surgical oophorectomy or amenorrhoea >= 5 years;
  • Age >= 56 years old and amenorrhoea >= 1 year;
  • Chemotherapy induced amenorrhoea >= 2 years;
  • Radiotherapy induced amenorrhoea at least 3 months before:
  • Age < 56 and < 5 years of amenorrhoea: follicle-stimulating hormone (FSH) levels to confirm postmenopausal status.
• Metastatic breast cancer (stage IV) or non-operable locally advanced breast cancer (stage IIIB).
• Positive estrogen and/or progesterone receptors as >10% cells or >10fmol/mg.
• Measurable disease as per Response Evaluation Criteria in Solid Tumors (RECIST) criteria.
• Patients who have received adjuvant tamoxifen are eligible, if progression has been established at least 24 months since treatment start.
• Neoadjuvant chemotherapy is allowed if progression has been established at least 12 months after end of treatment.
• Patients may have received a first line of chemotherapy for advanced disease, but treatment must have ended at least 4 weeks before enrolment, and all acute toxicities must be resolved. Previous treatment with Herceptin is allowed.
• Normal haematological, hepatic and renal functions.
• Performance status ECOG of 0, 1, 2.
• Life expectancy superior to 3 months.
• Written informed consent.

Exclusion Criteria:

• Previous hormone treatment for metastatic disease.
• Previous treatment with aromatase inhibitors.
• Inflammatory breast cancer, or aggressive metastatic disease, or visceral lesions, or metastasis in the central nervous system (CNS).
• Non-measurable disease.
• Second malignancy except for basal skin carcinoma or cervical in situ carcinoma adequately treated. If other malignancies, patient must have a disease-free period superior to 5 years.
• Treatment with any investigational product in the 4 previous weeks.
• Patients with negative estrogen and progesterone receptor tumours.