&NA;The authors have investigated the frequency with which the rectum contains feces by recording the presence or absence of fecal contamination of the membrane or of discoloration by feces of the contents of dialysis bags placed in the unprepared rectum for one hour. Feces were present in the rectum in 31 per cent of 32 studies in normal control subjects, in 49 per cent of 80 studies in obese subjects (P<0.05 from controls), in 36 per cent of 28 studies in patients with the irritable bowel syndrome, and in 31 per cent of 103 studies in patients with ulcerative proctocolitis, whether or not they had diarrhea. Fecal staining of the bag and its content occurred much more frequently in 27 studies in subjects with various other diarrheal diseases (67 per cent,P<0.02 from controls), including eight with stearorrhea (87 per cent,P<0.02 from controls). The frequency with which feces were present was unaffected by age, sex, or time of day of the study. These results provide quantitative support for the assertion that in subjects without diarrhea the rectum is usually empty. In patients with diarrhea or steatorrhea and no distal large intestinal inflammation, however the rectum usually does contain fecal material.