The present study is based on the assumption of an interaction between family functioning and chronic illness. Using a systemic approach, the intra-familial situation of families with a diabetes-affected child is examined. 44 families were evaluated using a family diagnostic instrument ("Familienbögen") and compared with 31 control families with a healthy child. Furthermore, the study looked at the influence of the level of family functioning on glycemic control, as measured by HbA1c values, and vice versa. Families with a child affected by diabetes showed significantly more dysfunctional domains and higher discrepancies of the ratings in the family diagnostic instrument (p < 0.05). Unexpectedly, no significant interaction between family functioning and glycemic control was found. Poor glycemic control therefore did not have any negative effects on the family dynamics, in fact, the opposite was often the case. Also, the relationship between siblings was judged more positively when one of the siblings was chronically ill (p < 0.05). The results show that despite the fact that diabetes in children may lead to an increase in impairment of intra-familial dynamics, it may, at the same time, offer opportunities for an improvement of family relationships. However, if physiological parameters deteriorate in the child (poor glycemic control), family problems seem to become less important. Success in the treatment of diabetes patients should therefore not only be measured by the quality of glycemic control, but also by considering psychological factors and aspects of family dynamics.