This long-term prospective study examined whether illness self-concept (or the degree to which chronic illness becomes integrated in the self) mediated the pathway from self-esteem to problem areas in diabetes in emerging adults with Type 1 diabetes. Having a central illness self-concept (i.e. feeling overwhelmed by diabetes) was found to relate to lower self-esteem, and more treatment, food, emotional, and social support problems. Furthermore, path analyses indicated that self-esteem was negatively related to both levels and relative changes in these problem areas in diabetes over a period of 5 years. Illness self-concept fully mediated these associations.