In some patients with faecal incontinence due to an obstetric tear of the external anal sphincter there is additional weakness of the anal sphincter muscles from damage to the innervation of these muscles during delivery. Of 19 patients who required surgical repair of an obstetric sphincter tear some months or years after injury, 9 (47 per cent) had evidence of pudendal nerve damage at pre-operative anorectal physiological investigation. The result of surgical repair was excellent or good in eight of the ten patients in whom there was no evidence of nerve damage, while this was the case in only one of the nine patients with nerve damage. These results are significantly different (P = 0.018). Thus the functional result of delayed anal sphincter repair after obstetric lesions is partly dependent upon whether the nerve supply is intact. Pre-operative physiological evaluation can give information on the probability of a successful surgical result.