OBJECTIVE: To assess maternal, newborn, and obstetric risk factors associated with anal sphincter tear in multiparous women. METHODS: This case-control study identified 18,779 multiparous vaginal deliveries from 1992 to 2004 from an obstetric automated record database at the University of Alabama at Birmingham. Two hundred eighty-four patients were selected, 145 cases and 139 controls. Variables from the index pregnancy and prior pregnancies were analyzed, and multivariable logistic regression models were constructed to determine significant predictor variables for anal sphincter tear in multiparous women. RESULTS: One hundred forty-five multiparous women with no history of cesarean delivery sustained a sphincter tear. Multivariable logistic regression showed a significant association with episiotomy (odds ratio [OR] 16.3, 95% confidence interval [CI] 7.7-34.4), shoulder dystocia (OR 7.9, CI 1.6-38), forceps delivery (OR 4.7, CI 2.0-11.2), and being married (OR 2.2, CI 1.1-4.6). A second exploratory model that included variables from previous pregnancies, showed that in addition to episiotomy (OR 34.6, CI 8.8-136), shoulder dystocia (OR 11.1, CI 1.3-95.2), forceps delivery (OR 6.1, CI 1.6-23.5), previous sphincter tear (OR 7.7, CI 1.2-48.7), and second stage of labor greater than 1 hour (OR 6.7, CI 1.1-42.5) were associated with tear. CONCLUSION: The strongest clinical risk factors for anal sphincter tear in multiparous women are episiotomy, shoulder dystocia, previous sphincter tear, prolonged second stage of labor, and forceps delivery. © 2006 by The American College of Obstetricians and Gynecologists. Published by Lippincott Williams & Wilkins.