It is proposed that vitamin D is a protective factor against colon cancer. This hypothesis arose from inspection of the geographic distribution of colon cancer deaths in the U.S., which revealed that colon cancer mortality rates were highest in places where populations were exposed to the least amounts of natural light--major cities, and rural areas in high latitudes. The hypothesis is supported by a comparison of colon cancer mortality rates in areas that vary in mean daily solar radiation penetrating the atmosphere. A mechanism involving cholecalciferol (vitamin D3) is suggested. The possibility that an ecological fallacy or other indirect association explains the findings is explored.