This study will assess optimal duration of pre-operative treatment with letrozole and to correlate clinical efficacy with appropriate surrogate markers.

Inclusion criteria:

1. Female Breast Cancer patients with primary invasive breast cancer, histologically confirmed by core needle biopsy, whose tumors are estrogen (ER) and / or progesterone (PgR) positive (Clinical Stage *T2, T3, T4a,b,c, N0, M0)
2. Patients after menopause
3. Tumor measurable by clinical examination, mammography and ultrasound
4. Adequate bone marrow, renal and hepatic function
5. A life expectancy of at least 6 months.

Exclusion criteria:

1. Prior treatment with aromatase inhibitors or antiestrogens.
2. Uncontrolled endocrine disorders such as diabetes mellitus, confirmed hypo- or hyperthyroidism, Cushing´s Syndrome, Addison´s disease (treated or untreated), Patients with unstable angina, uncontrolled cardiac disease (e.g. Class II or IV New York Heart Association's Functional Classification).
3. Patients with tumors in both breasts, evidence of inflammatory breast cancer or distant metastasis
4. Patients who are eligible for breast conserving surgery.
5. Concomitant anti-cancer treatments such as chemotherapy, immunotherapy/biological response modifiers (BRM's), endocrine therapy (including steroids), bisphosphonate therapy and radiotherapy. Patients who have received HRT will NOT be excluded, provided that HRT is discontinued at least 2 weeks prior to entry into the study.

Other protocol defined inclusion/exclusion criteria may apply