Elementary school children in a large public urban school system (Chicago) can be tracked into adolescence, together with their families, by using student numbers established by the Chicago Public Schools. This paper reports on the linkage between a psychiatric follow-up study and the data bank of the Chicago Public Schools. The authors were able to find information about the location and grade placement of 87 per cent of an urban ghetto neighborhood's first Both longitudinal and cross-sectional, epidemiologic studies are of importance for psychiatry, and for other health and education research, but to some degree each excludes the other. Longitudinal study requires long-term maintenance of contact with cases, an expensive and difficult process, forcing researchers to work with relatively few cases for long-term reassessment. Cross-sectional, epidemiologic studies, however, require enough cases to assure representativeness, making frequent contact too expensive. The problem for epidemiologic follow-up studies, therefore, is how to find large numbers of cases after a long period of time when there has not been frequent contact.In this paper we describe a method by which we have retrieved a large population of children for long-term psychiatric reassessment as part of an epidemiological follow-up after nine to ten years. We shall also describe relations between early mental health measures and retrievability, and between these measures and later grade level-one index of outcome.The development of the ecological theory of urbanization in the 1920s made possible such studies as those of Address reprint requests to Dr.