Objective. To identify the risk factors for obstetric anal sphincter rupture (OASR). Design and setting. Retrospective population-based register study. Population. A total of 514,741 women with singleton pregnancy and vaginal delivery between 1997 and 2007 in Finland. Methods. 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. Main outcome measure. The OASR risk. Results. 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). Conclusions. The results support the restrictive use of episiotomy, since 909 episiotomies appear to be needed to prevent one OASR among primiparous women. Equivalent estimate in vacuum assisted deliveries among primiparous women was 66, favoring routine use of episiotomy in such cases.