Turning a nation into a cohort: combining national registers and a digital mailbox for population-based health research.

Denmark has long been internationally recognized for its comprehensive national registers, which have supported population-based health research across medical specialties and enabled studies in which the entire national population can function as a single cohort through person-level linkage. These registers have provided rich longitudinal data on healthcare use, diagnoses, treatments, and outcomes, forming a robust foundation for observational research. The introduction of the national digital mailbox system, Digital Post, has added a powerful new dimension to this established research infrastructure. By enabling direct, secure, and authenticated communication with individual citizens, Digital Post extends register-based research beyond passive data collection to active engagement with the population. Because Digital Post is linked to a unique personal identification number, the Central Person Register (CPR) number, and access is protected by a national digital identity key, researchers can precisely define study populations within registers and contact eligible individuals at a nationwide and scalable level. This integration makes it possible to collect patient-reported outcomes and other self-reported data at a national scale and at low cost, complementing longitudinal register data, and facilitating recruitment of participants for research studies. This article describes the current Danish register-based research landscape and demonstrates how the addition of Digital Post expands its methodological possibilities.