We investigated the psychosocial effects of bone marrow transplantation (BMT) on siblings of transplant recipients. We asked how donor siblings compared with nondonor siblings on quantitative measures of behavior, psychological distress, and sense of self. Participants included 44 siblings (21 donors and 23 nondonors, ages 6–18 yr) of surviving pediatric BMT patients. On self-report measures, donors reported significantly more anxiety and lower self-esteem than nondonors. On teacher-rated scales, donors showed significantly more adaptive skills in school. On these same scales, nondonors showed significantly more school problems than donors. One-third of the siblings in each group reported a moderate level of post-traumatic stress reaction. Exploratory multiple regression analyses point to factors that might influence sibling adjustment and suggest counseling strategies and avenues for future research.