Risk factors for colon cancer have essentially been considered in terms of relative risks. From a public health viewpoint, however, their impact depends not only on the strength of the association, but also on the distribution of exposures in the population. Thus we used data from a case-control study conducted in Italy between 1992 and 1996 to estimate the population-attributable risks (PARs) for colon cancer in relation to educational level, physical activity, energy and vegetable intake, eating frequency, and family history of colorectal cancer. Cases were 1,225 incident, histologically confirmed colon cancer patients admitted to the major teaching and general hospitals in six Italian areas; controls were 4,154 subjects with no history of cancer, admitted to hospitals in the same catchment areas for acute, nonneoplastic diseases. By use of the distribution of the risk factors in the cases and the multivariate relative risk estimates, PARs were computed, i.e., the proportion of colon cancer that would have been avoided if all subjects were moved to the lowest exposure level. The PARs were 12% for high education, 14% for low physical activity, 14% for high energy intake, 22% for low vegetable intake, 7% for high eating frequency, and 8% for a family history of colorectal cancer. These factors together accounted for 56% of colon cancer cases. PARs were similar across age strata. Men had higher PARs for education, physical activity, and their combination, but lower PARs for energy, eating frequency, vegetable intake, and their combination than women. The percentage of colon cancers attributable to all factors considered together was 50% in men and 67% in women. Even if the PAR estimates were based on several arbitrary assumptions on the exposure distribution for various risk factors, available knowledge could, in principle, explain > 50% of cases in this Italian population, thus indicating and quantifying the theoretical scope for prevention.