Vitamin D is available in two forms, vitamin D2 and vitamin D3. It has previously been assumed that these two forms maintain blood vitamin D equally. However, this may not be the case. This study will evaluate whether D2 and D3 produce equal elevation of blood vitamin D. Additionally, it will evaluate whether once per month vitamin D dosing is as effective in maintaining blood vitamin D levels as daily dosing.

Inclusion Criteria:

1. Community dwelling men and women age ≥ 65 years.
2. Able and willing to sign informed consent.
3. Serum 25OHD concentration ≥ 10 and less than 60 ng/ml by HPLC.
4. Willing to avoid use of cod-liver oil and non-study vitamin D supplementation; standard multiple vitamins containing ≤ 400 IU used no more than once daily will be allowed

Exclusion Criteria:

1. Current hypercalcemia (serum calcium > 10.5 mg/dl) or untreated primary hyperparathyroidism.
2. History of nephrolithiasis.
3. Screening 25OHD concentration ≥ 60 ng/ml.
4. Baseline 24-hour urine calcium > 250 mg if female, > 300 mg if male.
5. Known risk factors for hypercalcemia, e.g., malignancy, tuberculosis, sarcoidosis, Paget's disease
6. History of any form of cancer within the past five years with the exception of adequately treated squamous cell or basal cell skin cancer.
7. Renal failure defined as a calculated creatinine clearance (Cockroft-Gault method) ≤ 25 ml/minute
8. Severe end-organ disease, e.g., cardiovascular, hepatic, hematologic, pulmonary, etc., which may limit ability to complete the study
9. Known malabsorption syndromes, e.g., celiac disease, radiation enteritis, active inflammatory bowel disease, etc.
10. Use of medications known to alter bone turnover including bisphosphonates, estrogen, selective estrogen receptor modulators, PTH, testosterone or calcitonin
11. Vitamin D intake greater than 5,000 IU daily
12. Treatment with any active metabolites of vitamin D within six months of screening
13. Treatment with any drug which may interfere with vitamin D metabolism, e.g., phenobarbital, phenytoin.