Defecography is commonly used in investigation of pelvic floor and anorectal dysfunction, and incorporates measurement of pelvic floor movement during various maneuvers. These measurements are usually referenced to bony landmarks, particularly the ischial tuberosities. These bony landmarks may be difficult to visualize; theoretically, the use of digital subtraction in filming defecography studies should eliminate the need to pinpoint bony position.We filmed 25 defecogram studies in both non-subtracted and subtracted formats, and interpreted each study blindly, subsequently comparing diagnoses and measurements.Subtraction was of limited benefit in only one case, was impossible in one case, added no useful information in 18 cases, and hindered visualization of abnormalities in five cases.Because of the multiple overlapping densities and the inability to restrict patient movement, digital subtraction is unhelpful in defecography.