The purpose of this study is to determine whether Vitamin D supplementation reduces the symptoms of muscle stiffness and joint tenderness that some patients may develop after starting therapy with Anastrozole for breast cancer.

This is a pilot, double-blind, randomized, placebo-controlled study with the following aims:

• To compare the effects of high dose vitamin D versus standard dose vitamin D on musculoskeletal pain in breast cancer patients receiving anastrozole when vitamin D levels are in the insufficient but not deficient range (10 to 29 ng/ml).
• To evaluate the prevalence of vitamin D insufficiency/deficiency in post-menopausal women with a history of breast cancer who are treated with anastrozole and have musculoskeletal symptoms versus those who are asymptomatic.
• To establish correlations between levels of vitamin D, the levels of PTH, the degree of bone loss and the severity of musculoskeletal symptoms.
• To evaluate the effect of standard dose versus high dose Vitamin D replacement on bone density in post-menopausal women with a history of breast cancer who are treated with anastrozole and have musculoskeletal symptoms.

Inclusion Criteria

• Women with a diagnosis of hormone receptor positive invasive breast cancer (Stage I-IIIB) or ductal carcinoma in situ (DCIS)
• Postmenopausal status
• Completed at least 8 weeks of anastrozole as adjuvant therapy prior to study entry
• Completed systemic chemotherapy and radiation treatments when indicated
• Serum Calcium ≤ 10.3 mg/dL
• Patients with marginal 25 OH Vitamin D level (between 10 and 29 ng/ml) will be in the Randomized Group or Low 25 OH Vitamin D level (less than 10 ng/ml) will be in Observational Group
• 24-hour urine Calcium excretion ≤ 250 mg/g (calculated by dividing 24 hour calcium by 24 hour creatinine)
• A history of generalized musculoskeletal pain with or without localized regions or discomfort that has developed or worsened since starting adjuvant aromatase inhibitor therapy

Exclusion Criteria

• Known metastatic disease
• History of kidney stones
• History of active primary hyperparathyroidism
• History of Paget's disease of the bone
• History of severe arthritis, rheumatoid arthritis, or severe neuropathy
• Normal 25 OH Vitamin D level (≥ 30 ng/ml)
• Medical or psychiatric condition which may preclude protocol compliance