Big data are today central for practices in disparate arenas such as science, public policy, administration, and business.Big health data are used to evaluate and plan public health policies as well as primary health care, and thus affect how people live their lives. Health and population registers – some of which go back to the eighteenth century in Sweden – are routinely used as sources of data. The availability of comprehensive – digitized – population records and the possibility of linking these to individuals and health data collections has laid the foundation for a virtual scientific success story where Sweden and the Nordic countries stand out as world-leading. Swedish and Nordic big health data appear as natural resources, ready to be mined. This development took off in the 1990s but has a longer history. The purpose of this paper is to examine historically the emergence, evolution and contents of the Swedish system of big health data. Firstly, I will map the registers that form part of the system and, secondly, discuss when, how, and involving which actors, they transformed from mainly being used as administrative tools to becoming irreplaceable, competitive instruments of global and national public health (and economic) value.