A consecutive series of 58 patients with idiopathic constipation and 20 control subjects were studied by evacuation proctography and measurements were made of changes during rectal expulsion. A wide range was found in the control group. The anorectal angle, pelvic floor descent, and the presence or size of an anterior rectocele did not discriminate between the control and patient groups. Internal intussusception was rare. Among constipated patients, the only significant differences from normal were in the time taken to expel barium and the amount of barium remaining in the distal rectum. The majority of control subjects (15 of 20) evacuated most of the barium within 20 seconds whereas 45 of 58 constipated patients took a longer time. Using the area of barium on a lateral view of the rectum as a measure, 19 of 20 control subjects evacuated at least 60 percent of the barium from the distal 4 cm of the rectum compared with only 25 of 58 patients. A varying degree of defecatory impairment was thus established among many patients with constipation. The patients were subdivided into those with a normal or abnormal whole gut transit rate as an indication of colonic function, and those who did or did not need to digitally evacuate the rectum as a clinical manifestation of an anorectal disorder. No obvious differences were found between these subgroups using the parameters measured.