Objective. To study changes in the incidence of obstetric anal sphincter rupture (OASR) during recent years in Denmark, Finland, Sweden and Norway and hospital-based incidence in recent years in Norway. Design. Retrospective birth register study. Setting. Unselected population of delivering women in four Nordic countries. Sample. All deliveries (574 175) registered in Denmark, Finland, Norway and Sweden, 2004–2010. Methods. Parity data, including maternal, obstetrical and fetal characteristics, were obtained. The incidence of OASR was calculated from vaginal deliveries. A chi-squared test was used to analyse differences between countries and time periods. Main outcome measures. Incidence of OASR. Results. During the study period, the OASR incidence in Finland was notably lower (0.7–1.0%) than in the other three Nordic countries (4.2–2.3%). A significant and constant reduction in OASR incidence was observed in Norway only (from 4.1 to 2.3%, from 2004 to 2010, p < 0.001). This reduction occurred simultaneously with introduction of a national intervention program of improved delivery techniques that aimed to reduce the incidence of OASR. No major alterations in maternal or fetal risk factors for OASR or registration routines could explain this rapid reduction in the rate of OASR. Differences in the incidence of OASR between Norwegian delivery units were significant, with a threefold difference when comparing the units with lowest and highest incidences. Conclusions. Obstetric anal sphincter rupture seems to be preventable to a considerable extent, as indicated by the rapid and lasting reduction of OASR incidence after implementation of perineal protection programs in Norway. Improved delivery techniques should be implemented in all delivery units to prevent OASR as much as possible.