['To identify the risk factors for obstetric anal sphincter rupture (OASR).', 'Retrospective population-based register study.', 'A total of 514,741 women with singleton pregnancy and vaginal delivery between 1997 and 2007 in Finland.', 'Primiparous (n = 2,315) and multiparous women (n = 534) with OASR were compared with primiparous and multiparous women without OASR by using stepwise logistic regression analysis.', 'The OASR risk.', 'Episiotomy decreased the likelihood of OASR for the primiparous [odds ratio (OR) 0.83, 95% CI (confidence interval) 0.75-0.92], but not the multiparous women (OR 2.01, 95% CI 1.67-2.44). The strongest risk factors for OASR among the primiparous women were forceps delivery (OR 10.20, 95% CI 3.60-28.90), birth weight over 4,000 g (OR 4.66, 95% CI 3.86-5.63), vacuum assisted delivery (OR 3.88, 95% CI 3.25-4.63), occiput posterior presentation (OR 3.17, 95% CI 1.64-6.15), and prolonged active second stage of birth (OR 2.06, 95% CI 1.65-2.58). Episiotomy was associated with decreased risks for OASR in vacuum assisted deliveries (OR 0.70, 95% CI 0.57-0.85). Risk factors for OASR among the multiparous women included forceps delivery (OR 10.13, 95% CI 2.46-41.81), prolonged active second stage of the birth (OR 7.18, 95% CI 4.32-11.91), birth weight over 4,000 g (OR 5.84, 95% CI 3.40-10.02), and vacuum assisted delivery (OR 4.17, 95% CI 3.17-5.48).'] Does lateral episiotomy protect primiparous but not multiparous women from obstetric anal sphincter rupture?