Anal endosonography was performed in 62 consecutive patients with incontinence of flatus or faeces following obstetric trauma, and in 18 parous controls. Of the incontinent group, 90% had defects in the external sphincter, 65% in the internal sphincter and 44% disruption of the perineal body, compared with none of the controls. This triad of lesions is pathognomonic of obstetric trauma. Anal endosonography revealed a higher prevalence of sphincter damage than expected from anorectal physiology tests, and therefore has a role in screening patients following complicated or difficult deliveries.