Background & Aims: The pathophysiology of constipation in the syndrome of obstructed defecation is unknown. Using 24-hour pancolonic manometric recordings of the unprepared colon to record basal pressures and spontaneous defecation episodes,we tested the hypothesis that the frequency, timing, or spatial distribution of propagating colonic pressure waves is abnormal in patients with obstructed defecation. Methods: In 11 patients with obstructed defecation and 16 healthy controls,pressures were recorded using a nasocolonic catheter that was positioned such that 16 recording sites spanned the unprepared colon at 7.5cm intervals. Results: The overall frequency of propagating sequences (PS) in the colon did not differ between patients and controls. When compared with controls, patients had a significant increase in the frequency of retrograde and antegrade PS (P< 0.05) in the left colon and a significant reduction in the amplitude of propagating pressure waves throughout the entire colon (P < 0.03). Defecation occurred in 6 of 11 patients and 9 of 16 controls. In the 15 minutes before defecation,controls showed a highly significant increase in frequency (P= 0.001) and amplitude (P = 0.01) of PS. In contrast, patients did not demonstrate this or the typical spatiotemporal organization of PS normally observed before expulsion of stool.Conclusions: Patients with obstructed defecation lack the normal predefecatory augmentation in frequency and amplitude of propagating pressure waves and lack the normal stereotypic spatiotemporal patterning of colonic pressure waves that would normally culminate in effective expulsion of stool. Background & Aims: The pathophysiology of constipation in the syndrome of obstructed defecation is unknown. Using 24-hour pancolonic manometric recordings of the unprepared colon to record basal pressures and spontaneous defecation episodes, we tested the hypothesis that the frequency, timing, or spatial distribution of propagating colonic pressure waves is abnormal in patients with obstructed defecation. Methods: In 11 patients with obstructed defecation and 16 healthy controls, units were recorded using a nasocolonic catheter that was positioned such that 16 recording sites spanned the unprepared colon at 7.5 cm intervals. Results: The overall frequency of propagating sequences (PS) in the colon did not differ between patients and controls. When compared with controls, patients had a significant increase in the frequency of retrograde and antegrade PS (P <0.05) in the left colon and a significant reduction in the amplitude of propagating pressure waves throughout the entire colon (P <0.03). Defecation occurred in 6 of 11 patients and 9 of 16 controls. In the 15 minutes before defecation, controls showed a highly significant increase in frequency (P = 0.001) and amplitude (P = 0.01) of PS. In contrast, patients did not demonstrate this or the typical spatiotemporal organization of PS normally observed before expulsion of stool. Conclusions: Patients with obstructed defecation lack the normal predefecatory augmentation in frequency and amplitude of propagating pressure waves and lack the normal stereotypic spatiotemporal patterning of colonic pressure waves that would normally culminate in effective expulsion of stool.