To identify incidence and risks factors of obstetrical anal sphincter lacerations, we reviewed all cases of 3rd and 4th degree sphincter lacerations after vaginal deliveries (VD) occured in a tertiary maternity between 2005 and 2010. 78 anal sphincter lacerations were identified (3.8/1000 deliveries). 66 women (85%) were nulliparous. The mean age of women was of 29 +/- 5 years. The mean duration of the second stage of labour was of 3.4 +/- 1,7 hour. The birth weight of 12 newborns (15%) was greater than 4000 grams. Fourty-six women (2.5%) had sphincter lacerations after instrumental delivery and 32 after spontaneous vaginal delivery (0.2%). 51 patients out of 78 (65%) had a medio-lateral episiotomy, 82% occured after instrumental extraction and 43% after spontaneous delivery. A forceps of Tarnier was used in 18 cases, a Suzor forceps in 12 cases, spatula in 12 cases and vacuum in 4 cases. 76% of foetuses were in anterior presentation and 78% at the medium part of the pelvis. 2 patients experienced anal incontinence in early post-partum. Anal sphincter lacerations are relatively frequent after VD especially after instrumental delivery. Macrosomia, nulliparous women, prolonged second stage of labor were associated with anal sphincter tears. Medio-lateral episiotomy does not protect enough anal sphincters. Anal sphincter lacerations lead to anal incontinence in some cases.A long follow-up is useful for these patients.