The problem of boundaries between chronically ill adolescents and their mothers is reviewed from psychoanalytic and developmental perspectives. In general, the transition to adulthood encompasses several processes that are especially important for chronically ill adolescents, including the differentiation and fusion of self and others, the integration of the mature genitals into the body concept, and the acceptance of a defective body. In particular, the constant vigilance over and care of the body prescribed for adolescents afflicted with diabetes, as well as the necessity of repeatedly injuring their bodies by injecting insulin, have no correlate in normal development. The difficulties of balancing the need for boundaries and the wish for fusion and merger are illustrated by a case study from a longitudinal study on diabetic and healthy adolescents and their families.