Blood samples taken in 1974 in Washington County, Maryland, from 25 620 volunteers were used to investigate the relation of serum 25-hydroxyvitamin D (25-OHD) with subsequent risk of getting colon cancer. 34 cases of colon cancer diagnosed between August, 1975, and January, 1983, were matched to 67 controls by age, race, sex, and month blood was taken. Risk of colon cancer was reduced by 75% in the third quintile (27-32 ng/ml) and by 80% in the fourth quintile (33-41 ng/ml) of serum 25-OHD. Risk of getting colon cancer decreased three-fold in people with a serum 25-OHD concentration of 20 ng/ml or more. The results are consistent with a protective effect of serum 25-OHD on colon cancer.