The purpose of this study was to define the self‐concept of children with cancer in comparison with healthy children and children with thalassaemia. MaterialMethod: 165 children with cancer were studied and compared to 417 healthy children and 212 children with thalassaemia of corresponding age. The children of the study and the controls with thalassaemia were drawn from hospitalized children at two of the largest paediatric hospitals in Athens – ‘Pan. and Agl. Kyriakou’ and ‘Agia. Sophia’. The healthy controls came from schools in Athens. The collection of information was accomplished by interview between the researcher and children of all three groups. For all groups of children the Piers‐ Harris scale ‘How do I feel about myself’ was used as a tool. The results of the data analysis showed that children with cancer have a negative image of themselves in comparison to the controls. In a further analysis, they demonstrate behavioural problems (p<0.001), especially children below the age of twelve. Their sickness hinders their correct intellectual and academic development (p<0.001) and they feel less happy and less satisfied (p<0.001) than healthy children of the same age. In conclusion the illness and its treatment affect the self‐concept of children negatively and according to the results, the children with cancer evaluate their behaviour, appearance, performance at school negatively and express less satisfaction and happiness.