Post-menopausal breast cancer patients will receive letrozole 2.5 mg daily for the treatment of breast cancer and will be randomized to a treatment group to receive either upfront zoledronic acid 4 mg IV 15-minute infusion every 6 months or delayed start zoledronic acid 4 mg IV 15-minute infusion every 6 months. Delayed start zoledronic acid will be initiated when either the Bone Mineral Density T-score is below -2 Standard Deviations at either the lumbar spine or hip or any clinical fracture unrelated to trauma or an asymptomatic fracture discovered at the month 36 scheduled visit. Letrozole 2.5 mg will be given daily for 5 years.

Inclusion Criteria:

• Stage I-IIIa breast cancer
• Postmenopausal or recently postmenopausal
• Recent surgery for breast cancer
• Estrogen Receptor positive and/or progesterone receptor positive hormone receptor status
• No prior treatment with letrozole

Other protocol-defined inclusion criteria may apply.

Exclusion Criteria:

• Metastatic disease
• Invasive bilateral disease
• Clinical or radiological evidence of existing fracture in spine or hip
• Prior treatment with IV bisphosphonates in the past 12 months
• Current treatment with oral bisphosphonates ( must be discontinued within 3 weeks of baseline evaluation)
• Use of Tibolone within 6 months
• Prior use of parathyroid hormone for more than 1 week
• Previous or concomitant malignancy
• Abnormal renal function
• History of disease effecting bone metabolism

Other protocol-defined exclusion criteria may apply.