The aim of this study is to gain more insight into basic aspects of identity, in relation to adolescent chronic fatigue syndrome (CFS) and juvenile idiopathic arthritis (JIA). In dialogical self theory, identity is regarded as incorporating multiple self-positions, such as 'I as tired', 'I as pessimistic', or 'I as decisive'. Physical and psychosocial impairment might alter the organization of these self-positions. The Personal Position Repertoire procedure, a quantitative method to analyse the prominence of self-positions, the Child Health Questionnaire, assessing health-related functioning, and the Checklist Individual Strength, measuring fatigue, were completed by 42 adolescents with CFS, 37 adolescents with JIA and 23 healthy teenagers. Adolescents with JIA report impaired physical functioning and general health. However, they position themselves very similar to healthy teenagers - i.e. as strong and healthy. While this self-positioning approach might be adequate and sustainable in adolescence, it could prove too strenuous to maintain throughout adult life. Adolescents with CFS, besides indicating severe physical difficulties, also report more psychosocial problems. They position themselves as significantly less strong and more unwell. With this emphasis on positions relating to their illness, there seems to be little room left for stronger positions. It is regarded of clinical importance to address these issues in this crucial developmental period.