Increased public awareness of the possible presence of carcinogens and other potentially harmful agents in the workplace and in other areas of the environment has created a demand for studies to determine the extent of the risks associated with exposure to such agents. These studies require large numbers of individuals in various "control" populations to be followed-up over long periods of time. Such large-scale follow-up can be greatly facilitated where information on all deaths and on cases of serious morbidity is accumulated centrally, for a whole country, in a form that permits rapid searching by computer and in which individuals are well enough identified to minimize the possibility of mistaken identity. The Canadian Mortality Data Base and the National Cancer Incidence Reporting System are two such centralized follow-up facilities which have been developed in Canada. We describe here the manner in which these files are used, the problems encountered, and their solutions.