BACKGROUND: Pre-eclampsia, the etiology of which is unknown, is an important cause of maternal morbidity and mortality. In Norway there has been a substantial change in lifestyle since 1970. Akershus is a growing urban county. The aim of this study was to identify changes in the prevalence of pre-eclampsia in Akershus County and the rest of Norway during the past 35 years. METHODS: The study is population-based and retrospective. Deliveries are registered per inhabitant and pre-eclampsia per delivery. Prevalence and prevalence ratios are calculated with 95% confidence intervals. Poisson regression analysis is used for assessment of trends. RESULTS: The frequency of deliveries declined from 1.9% to 1.4% for Akershus County and from 1.7% to 1.3% in the rest of Norway. The prevalence of pre-eclampsia increased from 3.1% to 3.7% for Akershus County and from 3.3% to 4.5% in the rest of Norway. CONCLUSIONS: The increase in the prevalence of pre-eclampsia and the difference between Akershus and the rest of Norway are not likely to be explained by changes in genetic material. Altered registration routines throughout Norway since 1998 may influence the observations, and better registration routines may contribute to the observed lower prevalence in Akershus. Changes in lifestyle factors most likely explain the overall increase in the prevalence of pre-eclampsia.