This clinical trial is aimed at measuring the effect of vitamin D3 supplementation on serum prostate-specific antigen (PSA) levels in patients diagnosed with early stage, low-grade, low-risk prostate cancer (Gleason score less than/equal to 6; PSA less than/equal to 10; clinical stage T1C or T2a), who elect to have their disease monitored through active surveillance for at least one year.

Inclusion Criteria:

• lowgrade prostate cancer
• serum PSA less than/equal to 10 ng/ml
• Gleason score less than/equal to 6
• referred from their treating physician with treatment plan of active surveillance for one year
• serum creatinine less than/equal to 2.0 mg/dL
• serum phosphate (measured as phosphorus)greater than 2.3 and less than 4.8 mg/dL
• serum calcium greater than 8.5 and less than 10.5 mg/dL

Exclusion Criteria:

• concurrent malignancy, except non-melanoma skin cancer
• history of sarcoidosis
• history of high dose (greater than 1,000 IU daily) vitamin D3 supplementation
• history of hypercalcemia
• treatment with lithium medication