The authors performed a randomized clinical trial to determine the effect of flexible proctosigmoidoscopy (FPS) on the quality of air-contrast barium enema (ACBE) studies performed on the same day and whether it mattered if air or carbon dioxide was used for endoscopic insufflation. One hundred twenty-one patients were randomly assigned to one of the following groups: Same-day studies were performed, with air used for insufflation, in group 1; same-day studies, with carbon dioxide, in group 2; and separate-day studies in group 3. Scout images obtained before the ACBE study were graded for the amount of air seen, and ACBE studies were graded for overall quality; each was graded on a scale of 0-3 in a blinded fashion. The air score was significantly greater in group 1 (2.69) than in group 2 (2.01) (P less than .001), which in turn was significantly greater than in group 3 (1.53) (P less than .01). The ACBE quality scores in the three groups were not significantly different. The authors conclude that FPS can be done before ACBE examination without impairing the quality of the ACBE study. Although carbon dioxide insufflation results in less intestinal air after FPS, the quality of the ACBE study is not affected.