INTRODUCTION: The genetic and familial environmental influences making up familial correlations in traits and familial aggregation of diseases may be disentangled by adoption studies. Thus, resemblance between the adoptee and the biological relatives indicates a contribution of the segregating gene they have in common, and resemblance between the adoptee and the rearing adoptive family members indicates influences of the shared family environment. The Danish Adoption Register includes information on all 14,425 non-familial adoptions of Danish children legally granted in Denmark from 1924 to 1947. RESEARCH TOPICS: To illustrate the adoption study approach in research, we present results achieved in schizophrenia, body fatness, smoking, and mortality. Significantly higher prevalence of schizophrenia and related conditions were found in biological relatives to adoptees with schizophrenia than among biological relatives to adoptees without schizophrenia. Clear trends in body mass index of biological relatives and the weight of adoptees were found in several settings. A genetic influence on smoking habits were found within a generation. A moderate genetic influence on the mortality rate has been found. CONCLUSION: The Danish Adoption Register has contributed to the knowledge about environmental and genetic influence in various fields. Genetic influence on the risk of schizophrenia, body fatness, smoking habits, and on the mortality rate has been found.