Annual fecal occult blood test (FOBT) screening using rehydrated Hemoccult slides has been reported in the Minnesota Colon Cancer Control Study to reduce colorectal cancer mortality by about 33%. However, some of the benefit of FOBT screening may come from "chance" selection of persons for colonoscopic examination because of the high positivity rate of FOBT (about 10%) that may occur for reasons other than a bleeding cancer or polyp. To determine how much this mechanism could account for the benefit of FOBT screening, we used a simple mathematical model to simulate the course of a cohort of screened persons, incorporating published data including those from the Minnesota study. The results suggest that one third to one half of the mortality reduction observed from FOBT screening in the Minnesota study may be attributable to chance selection for colonoscopy. We conclude that annual FOBT screening with rehydration is a haphazard method for selecting persons for colonoscopy.