Abstract A balloon filled with barium has been used to simulate a soft stool for the radiological study of disorders of defaecation. Lateral radiographs demonstrate the level of the pelvic floor in relation to the pubococcygeal line, the change in the anorectal angle and the behaviour of the anal sphincters. In 12 patients with faecal incontinence, successfully treated surgically by postanal sphincter repair, the anorectal angle was reduced from 135°±4·4° (s.e.m.) to 103°±4·1° and the anorectal junction rose by 1·5±0·4 cm. In 10 patients with slow-transit constipation there was no pelvic descent and no change in the anorectal angle on straining. These patients were unable to expel the balloon and the results suggest that the pelvic floor does not relax normally.