We hypothesize that a daily dose of vitamin D3 (4,000 IU) taken for one year by Veterans diagnosed with low-risk, early-stage PCa, who are eligible for active surveillance will: a) result in a measurable decrease of serum PSA levels in a significant number of enrolled subjects, and b) be associated with a stabilization or improvement of their PCa pathology, as assessed through histological examination of prostate tissue biopsy specimens (Gleason score and percent of positive biopsies) obtained at the end of the study, as part of their standard medical care for active surveillance.