INTRODUCTION: Researchers in Denmark have unique possibilities of register-based research in relation to migration, ethnicity, and health. This review article outlines how these opportunities have been used, so far, by presenting a series of examples. RESEARCH TOPICS: We selected six registers to highlight the process of how migrant study populations have been established and studied in relation to different registers: The Danish Cancer Registry, the Danish Central Psychiatric Research Register, the Danish National Patient Register, the Danish National Health Service Register, the Danish Injury Register, and the Danish Medical Birth Register. CONCLUSION: Our paper documents the unique opportunities to study migration, ethnicity, and health through Danish national registers. Our examples show that in Denmark ''country of birth'' is the most commonly used measure. It renders information on whether the person is an immigrant or not, and on ethnic background. Data on migration background (i.e. refugee status vs. family reunification, etc.) is more difficult to obtain and therefore less used. It has been debated if ethnicity should be registered upon using health services; however, some consider it discriminatory. Although, we do not register ethnicity in relation to use of health care in Denmark, our possibilities of linkage between population registers and registers on diseases and healthcare utilisation appear to render the same potentials.