Vitamin D promotes the differentiation of prostate cancer cells, maintains the differentiated phenotype of prostate epithelial cells, and can induce prostate cancer cell death, raising the possibility that vitamin D deficiency over time promotes the progression of subclinical prostate cancer to clinical disease. The investigators propose to conduct a clinical study aimed at measuring the efficacy of vitamin D3 (4000IU/day) supplementation in Veterans diagnosed with low-risk, early-stage prostate cancer, who elect to have their disease monitored through active surveillance. The successful completion of this proposed clinical study will allow us to determine whether correcting vitamin D deficiency in Veterans diagnosed with early-stage prostate cancer will prevent progression of their disease and improve their prognosis.