Data from a health survey including the Hemoccult fecal blood test, together with official cause-specific death rates, were used to assess the magnitude of a controlled trial that would be required to prove a 25 percent reduction of the mortality from colorectal cancer associated with screening. All men in three age groups in the city of Malmö, Sweden, were invited, but 46 percent did not participate in, the Hemoccult screening. One carcinoma and 89 adenomas were detected in 56 of the 2422 who did. With the risk function used in our calculation and a compliance rate of 60 percent, a study population among 45- to 69-year-olds of 605,000 is required to prove an expected 25 percent reduction of the mortality with 90 percent power. Considering the size of such a trial, we question whether a controlled trial is feasible. With known risk functions for death from all causes and death from colorectal cancer, the study population was calculated using variable statistical power, participation rate, and risk reduction. Statistical methods and computer programs are given. In addition, alternative study models to assess the benefits associated with screening are discussed.