Many years detailed registration of the entire population in Denmark and Greenland has enabled the creation within our department of a familial database encompassing a total of 7.8 million persons. Information on present and past residential address permits detailed studies regarding whether possible effects may be explained by the sharing of environment or genetic constitution. By linking this resource with information from disease registries such as the national patient discharge we are presently studying a number of disease complexes for the possible existence of familial aggregation. One example of a question which is examined based on this approach is to what extent our immunologic responses is inherited. Other population-based activities regarding familial aggregation of disease are undertaking in Greenland. Here the focus is on genetic susceptibility to nasopharyngeal carcinoma and Epstein-Barr virus infection.