The incidence and outcome of third-degree tears following 16,583 vaginal deliveries were prospectively assessed over a 5.5-year period. Ninety-three deliveries (0.56 per cent) were complicated by a third-degree tear and the patients underwent primary repair. Eighty-one patients were reviewed 3 months postpartum at a colorectal clinic. Third-degree tears were significantly more common in primigravidae and mothers with higher birth-weight babies. They were significantly associated with the use of forceps and were not prevented by episiotomy. Of the 81 patients reviewed, 30 had an abnormal anorectal examination. Six patients (7 per cent) were incontinent of faeces. A further ten (12 per cent) were incontinent of flatus only. The overall incidence of faecal incontinence was 0.04 per cent. An important group of women with significant subclinical sphincter injury was identified. Obstetric trauma causes significant anorectal dysfunction and patients with third-degree tears require assessment by a colorectal specialist.