Lolland-Falster is a rural area in south-eastern Denmark that scores unfavourable in health surveys and has the lowest life expectancy in the country. To determine the origin of poor health in Lolland-Falster, we investigated impact on mortality of long-term population movements. Long-term selective in-migration of vulnerable persons was behind the gradual build-up of the currently high mortality in Lolland-Falster compared to the rest of Denmark. In particular, people of working age in-migrating to Lolland-Falster contributed to this disparity