Much research into spatial inequalities in health has sought the balance between compositional and contextual influences on observed patterns. Research published recently by the authors sought to determine whether composition of areas alone might account for the changing geography of mortality in Britain, between 1983 and 1993. The research required data describing Britain in terms of the numbers and location of people with every possible combination of age group, sex, social class and employment status. This paper describes the approach and the custom written geographic information system which estimated these data, and their subsequent application.