A between and within group design using a semi-structured interview was used to explore the content of children's illness representations of their asthma and the influence of these representations on knowledge, asthma management , asthma control and perceived quality of life. A sample of 41 asthmatic children and their parents were recruited from GP practices. Children were divided into two age groups 8-11 years and 12-16 years. Test-retest reliability was performed on a sample from each age group. Children were found to have similar illness representations across the age groups although developmental patterns were found in the beliefs children offered about the cause of their asthma, and in the way in which illness representations influenced outcome. For the younger children, causal attributions appear to be more influential in outcome than the constellation of illness representations that were tested Older children appeared to be beginning to approximate more adult patterns of holding a set of beliefs that influenced outcome. A major issue in the study was the variable reliability of the measures used Stability over time for the younger children was attributed to greater parental influence. The instability of measures over time for older children was considered to reflect developmental influences. Some support was found for the proposed self-regulatory function of illness representations although the complexity of findings highlights the need for longitudinal research to track developmental changes in children's illness representations. The implications for future research and clinical practice are discussed.