In a prospective study carried out on 78 patients with chronic constipation (31, with slow transit, 47 with obstructive defecation disorders) the evacuation function of the rectum during defecation was assessed by defecoflowmetry. These patients were compared to a control group of normal volunteers (n = 32). The following parameters were evaluated: defecation and retention volume, defecation fraction, defecation time, maximum flow, mean flow rate and time to maximum flow. As expected, there was no difference in evacuation function between the group of patients with slow transit and the control group. Significant differences, however, existed between the two types of constipation, as well as between obstructive defecation disease and controls, regarding all parameters mentioned above. Evacuation function depends neither on rectal neck pressure nor on intrarectal pressure. In patients with obstructive defecation disorders, three subgroups were discernable: one with prolonged time of defecation and satisfactory evacuation, one with prolonged time of defecation and poor evacuation, and one small group of patients who were not able to defecate. Each group is based on a different underlying pathomechanism. We conclude that changes in evacuation function of the rectum refer either to volume or to time of defecation, or to both. Changes are found only in obstructive type constipation, not in slow transit constipation. Therefore, defeconflowmetry as a dynamic procedure can be used in screening for the classification of chronic constipation.