To investigate the safety and efficacy of letrozole monotherapy at a dose of 2.5 mg/day in postmenopausal patients with breast cancer, and to determine the blood concentrations of letrozole and hormones.

Inclusion Criteria:

• Patients with histologically documented breast cancer.
• Patients with hormone receptor (ER and/or PgR) status of positive or unknown
• Patients who have been amenorrheic for the preceding 12 months or more.
• Patients who are 20 years or older and younger than 75 years.
• Patients with a history of postoperative adjuvant therapy or a history of endocrine therapy with tamoxifen for the treatment of progression or recurrence of the lesion. The endocrine therapy, however, should not exceed one regimen.
• Patients with progressing lesions.
• Patients with sufficient organ function to evaluate the safety
• Patients whose performance status (PS) is classified in 0～2.
• Patients who have no residual effects from previous treatments

Exclusion Criteria:

• Patients with other concurrent or previous malignant disease (excluding uterine carcinoma in-situ).
• Patients with hypercalcemia and uncontrollable cardiac disease (including a history of serious cardiac disease)
• Patients who have previously received aromatase inhibitor.
• Patients who have lymphangitis-type lung metastasis or symptomatic brain metastasis.

Other protocol-defined inclusion / exclusion criteria may apply.