OBJECTIVE: This study was conducted to identify obstetric risk factors for anal sphincter tear in primiparous patients, patients with a previous cesarean delivery (VBAC), and patients with a previous vaginal delivery (PVD). STUDY DESIGN: An obstetrics automated record system was accessed to retrospectively review records of all singleton vaginal deliveries at greater than 36 weeks' gestation (excluding breech and stillbirth) from 1995 through 2000 (n = 10,928). A number of potential risk factors for anal sphincter tear (third- and fourth-degree episiotomy extensions and lacerations) were tested with use of multivariate logistic regression analysis. RESULTS: The risk of anal sphincter tear was significantly increased with primiparity (relative risk [RR] 4.08) and VBAC (RR 5.46) compared with PVD, birth weight greater than 4000 g (RR 2.41), forceps delivery (RR 6.00), vacuum delivery (RR 2.18), shoulder dystocia (RR 3.28), and episiotomy (RR 2.59). CONCLUSION: Efforts to prevent anal sphincter tear might include reconsideration of modifiable risk factors such as episiotomy, operative vaginal delivery, and VBAC.