In this study 158 children, 80 children with diabetes mellitus and 78 healthy classmates, were interviewed about their concept of different types of illness (a cold, diabetes, infection, the most and least serious disease) and illness-related concepts (pain, becoming ill and going to the doctor or hospital). Special attention was given to the relationship between development of thinking and the variables anxiety, locus of control and family- and school functioning. The results show that the ideas of the children about the causes of illness follow a sequence of developmental stages, described as 'Through the Eyes of the Child' (TEC) model. Perception seems to be the child's central auto regulative system of cognitive development. The findings suggest that thinking about illness develops relatively independently of other influences. The practical relevance of knowing how children's thinking about illness develops is elaborated in terms of their implications for health education. Immature thoughts of children about illness can be detected and accepted and not dismissed as irrational. With the help of this model, health education of the child can be facilitated.