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.