Mortality rates from colon cancer in the USA are highest in populations exposed to the least amounts of natural sunlight; differences in endogenous vitamin D production and calcium absorption could be responsible. To investigate this possibility, the association of dietary vitamin D and calcium with 19-year risk of colorectal cancer was examined in 1954 men who had completed detailed, 28-day dietary histories in the period 1957-59. Risk of colorectal cancer was inversely correlated with dietary vitamin D and calcium. In the quartiles of a combined index of dietary vitamin D and calcium, from lowest to highest, observed risks of colorectal cancer were 38.9, 24.5, 22.5, and 14.3/1000 population. This association remained significant after adjustment for age, daily cigarette consumption, body mass index, ethanol consumption, and percentage of calories obtained from fat.