Colorectal cancer is the third most commonly occurring cancer in the United States and accounts for approximately 11% of cancer deaths. Many epidemiological studies have shown an association between dietary factors, including calcium and vitamin D, and the incidence of colon cancer. Recently the Calcium Polyp Prevention Study demonstrated that calcium supplementation can reduce the recurrence of colon polyps, but the effect depends on serum vitamin D levels. We used the Apc(min) mouse model of intestinal cancer to investigate the effects of vitamin D treatment and calcium intake independently on polyp formation. We found that 1,25-dihydroxycholecaliferol was potent in inhibiting tumor load; however, the dose used to achieve this antiproliferative effect led to deleterious effects on serum calcium homeostasis. These effects were minimized by use of a synthetic analogue with reduced toxicity. Additionally, we tested the effect of a modified-calcium diet in Apc(min) mice but did not find a protective effect, perhaps because of a reduction in circulating levels of 25-hydroxycholecaliferol with increasing levels of dietary calcium. A number of other studies that use rodent models with vitamin D supplementation or deficiency illustrate the efficacy of vitamin D in colon cancer prevention. The mechanisms of direct action of vitamin D on colonic epithelium include regulation of growth factor and cytokine synthesis and signaling, as well as modulation of the cell cycle, apoptosis, and differentiation. Because of the apparent synergistic effect of vitamin D and calcium, cosupplementation of both nutrients in cancer prevention programs may be advised.