The psychosocial care of children with cancer has received increasing and welcome emphasis in the medical literature. However, the current and future status of siblings of such patients has been an underestimated area of concern. We examine the general importance of sibling relationships and present the literature regarding siblings of paediatric oncology patients. Donors and recipients of bone marrow transplants are considered specifically. Preliminary results of original research, supported by available published studies, suggest important disturbances in the mental health of siblings. Implications for paediatricians and other health professionals as well as prevention and intervention strategies are indicated.