The Kaiser Permanente Multiphasic Evaluation Study is often cited as evidence from a randomized trial that screening sigmoidoscopy reduces mortality from colorectal cancer. To examine the role of sigmoidoscopy in this reduction, we reviewed the 110 incident cases of colorectal cancer occurring among the 10,713 subjects from randomization in 1964 through 1982. Tumor stage at diagnosis, location, mode of discovery, and current mortality status were determined for each. We also reanalyzed chart review data for the years 1965 through 1974 to assess the difference in exposure to sigmoidoscopy between groups. Study group subjects, who were urged to have annual multiphasic health checkups (MHC), had both a lower cumulative incidence (4.3 vs 6.7 cases per 1000 persons) and a better stage distribution (86 vs 54% Stage B or better) than nonurged control subjects for colorectal cancers arising within reach of the sigmoidoscope. The lowered incidence accounted for two-thirds of the total difference in mortality. No appreciable difference in removal of colorectal polyps was seen between groups. Only a slight excess in exposure to sigmoidoscopy was seen in the study group (30 vs 25% of subjects examined at least once between 1965 and 1974), which was unlikely to account for more than a small fraction of the study group's decrease in mortality. Although the Multiphasic Evaluation Study did find a significantly lower mortality from colorectal cancer in the study group, it did not achieve a substantial difference in exposure to sigmoidoscopy. Its results are therefore inconclusive with respect to sigmoidoscopy and should not be used as evidence either for or against sigmoidoscopic screening.