Although bone marrow transplantation (BMT) has become standard therapy for many life-threatening disorders of childhood, there is little research on the psychosocial impact of BMT on siblings of children undergoing BMT. Such siblings face issues common to any family with a chronic illness. However, the psychological impact on the family is intensified because two family members, usually children, are subjected to intrusive medical procedures. Investigators had earlier noted that sibling donors may be at risk for behavioral problems and anxiety, while nondonors may experience ambivalent feelings of disappointment and relief. It was suggested that psychosocial stages of BMT may parallel the medical transplant process, with high levels of stress experienced pre-BMT, during hospitalization, and post-discharge. Our own group has recently conducted more systematic investigations on the psychosocial effects of BMT on donor vs nondonor siblings of surviving pediatric BMT patients. We found that sibling donors showed significantly more anxiety, lower self-esteem, and more adaptive skills in school than nondonors. Nondonors, on the other hand, showed significantly more school problems. One third of the siblings in each group reported a moderate level of post-traumatic stress. Taken together, our research indicates that BMT affects the life of the child at home and at school and that post-traumatic stress symptomatology is a component of the psychological reaction in siblings. The psychosocial adjustment of siblings is a critical area of investigation in BMT populations. Parents need to know that the BMT process affects every member of the family system, and both parents and professionals need to direct more emotional support and attention to siblings. Studies are needed that focus on interventions designed to reduce levels of sibling psychosocial maladjustment. The psychosocial developmental model of post- traumatic stress disorder is a viable theoretical model that may be used to guide future research.