Nutritional factors are important determinants of colorectal cancer risk. Diets high in fat and/or low in fibre are especially recognised to increase risk. Dietary calcium and vitamin D have been suggested to be protective against colorectal cancer. With respect to calcium, its possible effect is thought to be mediated at least in part through intraluminal precipitation of hydrophobic, cytotoxic substances, in particular fatty and bile acids, which can promote colorectal cancer development. Data from studies in vitro and in animals support a protective effect of calcium, but studies in humans, both epidemiological and interventional, have given inconclusive results. With respect to vitamin D, data from only a small number of studies are available. Results suggest a protective effect by inhibition of cell proliferation, mediated through specific receptors. It is concluded that there are currently insufficient reasons to supplement subjects at increased colon cancer risk with calcium or vitamin D, especially when dietary intake of these substances is in agreement with general guidelines.