Twenty-two patients with functional obstructive defaecation were compared with seven patients with slow transit constipation using isotope proctography. The obstructive defaecation patients were unable to perform a complete rectal evacuation (% of the activity evacuated: 54%). The defaecation time in this group was prolonged (120 s) with a lower defaecation rate (0.5%/s) compared with the slow transit constipation subjects (32 s and 1.8%/s). The anorectal angle (ARA) on straining did not change in the obstructive defaecation patients but became more obtuse in the slow transit constipation subjects (P < 0.02). The pelvic floor (PF) descent on straining and evacuation was greater in obstructive defaecation patients compared with the slow transit constipation ones (P < 0.01). Obstructive defaecation is characterised by prolonged defaecation and reduced defaecation rate compared with slow transit constipation. Obstructive defaecation patients present with more acute ARAs on straining, and abnormal perineal descent in contrast to the slow transit constipation ones.