This paper examines the implications of occult fecal blood loss in patients taking aspirin (at least 2 grams daily). Although such patients do have a somewhat higher probability of colonic carcinoma than do members of the general population, their risk is far lower than that of patients who have gastrointestinal blood loss when not taking aspirin. This difference in risk exists because aspirin itself can provoke occult blood loss in stool. Patients who manifest gastrointestinal blood loss while taking aspirin can be separated into two groups, based on whether or not that blood loss continues after aspirin is discontinued. Although patients who continue to bleed are at high risk for colonic carcinoma, those who cease having any blood loss are at lower risk than are members of the general population. Further diagnostic studies to detect colonic carcinoma should be pursued in the former group, but not in the latter, low-risk group.