Hospitalized children (n = 60) in three age groups (5, 7, 9 years) were administered a number of tasks to explore their conceptions of illness and health and to assess development of physical and social concepts. A significant age progression was observed with these concepts, involving a shift from global, undifferentiated, to increasingly abstract principles. Children's conceptions of illness were also significantly related to performance in conservation, role taking, and physical causality tasks. The practical significance of this research in the context of the ill and hospitalized child is discussed.