Assuming that all colorectal cancers develop from preexisting adenomas, the annual conversion rate, defined as the number of cancers occurring each year as a percentage of all adenoma-bearing individuals, was determined. The number of adenoma-bearing individuals in the living population of northern Norway was estimated in each cohort for the period 1974-76 by determining the prevalence of colorectal adenomas in an autopsy population of 271 consecutive cases, representative of the population of the area with regard to underlying causes of death. During the 10-year period 1974-1983 a total of 656 colorectal cancers were recorded among an estimated number of 26,419 adenoma-bearing individuals aged over 35 years. The annual conversion rate was found to be 0.25%, indicating that an average adenoma-bearing individual is only at a moderate risk of colorectal cancer. The annual conversion rates for individuals having large adenomas, or adenomas with villous structures, or severe dysplasia were roughly estimated to be 3%, 17% and 37% respectively, assuming that colorectal cancer develops from one of these sub-groups of adenomas only.