&NA;The parameters of defecation,i.e., maximum rectal pressure increase during straining, duration of effective evacuation, and the work performed to evacuate a simulated stool, can be quantified by defecometry, a new method to evaluate the defecation act. Simultaneous anal pressure records demonstrate the nature of the sphincter activity during simulated defecation. The test was performed on 19 patients with constipation and on 14 controls. Five patients could not evacuate the simulated stool, while five others could, but more laboriously than the remaining nine patients whose defecation was comparable with the controls. Laborious defecation is characterized by longer duration and more performed work during evacuation. Every patient with difficult or ineffective evacuation had sphincter contraction during defecation, whereas this phenomenon was not observed in patients with normal defecation. Defecometry permits more adequate identification and characterization of the outlet‐obstruction‐type constipated patients than the simple balloon expulsion test and the analysis of sphincter activity during straining with empty rectum in lateral decubitus. Early diagnosis and treatment of patients with outlet obstruction is important to avoid late neuromuscular damage to the pelvic floor.