The association between episiotomy and severe (third- and fourth-degree) perineal lacerations was studied in 24,114 women. The overall rates of severe lacerations were 8.3 and 1.5% for primiparous and multiparous women, respectively. Women who had midline episiotomies were nearly 50 times more likely and women who had mediolateral episiotomies were over eight times more likely to suffer a severe laceration than were women who did not undergo an episiotomy. Severe lacerations were also more common after use of forceps, in occiput transverse and posterior presentations, among women with smaller pelvic outlet measurements or lower prepregnant weight, and with larger fetuses. The same factors that caused a woman to have an increased risk of laceration also made performance of an episiotomy more likely. After statistical adjustment for these risk factors, mediolateral episiotomy was associated with a 2.5-fold reduction in the risk of severe lacerations among primiparous women, and a statistically nonsignificant 2.4-fold increase among multiparous women, compared with no episiotomy. Midline episiotomy was associated with statistically significant 4.2- and 12.8-fold increases in the risk of lacerations among primiparous and multiparous women, respectively. We conclude that the risks and benefits of midline episiotomy should be evaluated in a randomized clinical trial that compares policies of "usual" versus conservative use of episiotomy.