This paper describes a study to investigate the relationship between self-esteem and behavioural adjustment in two groups of children with chronic illness, one with epilepsy and the other diabetes. A total of 62 children with epilepsy and 91 children with diabetes were recruited from the total population of children aged 8-15 attending the epilepsy and diabetic clinics at a children's hospital over a 12 month period. Self-esteem and behavioural adjustment were assessed with the Harter and Achenbach Questionnaires respectively. The results showed the children with epilepsy were consistently more behaviourally disturbed and had lower self-esteem than children with diabetes. The independent completion of the questionnaires, (the Harter by the child and the Achenbach by the parents) increases the validity of the findings. Long duration of illness was the most consistent illness variable associated with poor behavioural adjustment in the two groups. The cross-sectional design of the study did not make it possible to draw any definite conclusions about the causal or temporal relationship between low self-esteem and behavioural disturbance. Once again, the potential value of prospective studies into the psychosocial adjustment of children with chronic illness is highlighted.