Anxiety level, perception of control over illness, stage of cognitive development, and stage of reasoning about illness were examined in pediatric oncology, obese, and healthy children and adolescents. Among the groups, there were no mean differences with regard to any of these measures. Older participants in the higher stages of cognitive development were higher on stage of reasoning about illness in general and their own illness than were younger and less cognitively mature participants. In the pediatric oncology and obese groups, participants higher on perception of control over illness were higher on stage of reasoning about illness in general and their own illness, in particular, than those lower on perception of control. Anxiety level was not related to stage of reasoning about illness, but participants higher on anxiety were lower on perception of control over illness. Clinical implications of the findings are discussed.