OBJECTIVE: Sibling aggression is common but often dismissed as benign. We examine whether being a victim of various forms of sibling aggression is associated with children's and adolescents' mental health distress. We also contrast the consequences of sibling versus peer aggression for children's and adolescents' mental health. METHODS: We analyzed a national probability sample (n = 3599) that included telephone interviews about past year victimizations conducted with youth aged 10 to 17 or an adult caregiver concerning children aged 0 to 9. RESULTS: Children ages 0 to 9 and youth ages 10 to 17 who experienced sibling aggression in the past year (ie, psychological, property, mild or severe physical assault), reported greater mental health distress. Children ages 0 to 9 showed greater mental health distress than did youth aged 10 to 17 in the case of mild physical assault, but they did not differ for the other types of sibling aggression. Comparison of sibling versus peer aggression generally showed that sibling and peer aggression independently and uniquely predicted worsened mental health. CONCLUSIONS: The possible importance of sibling aggression for children's and adolescents' mental health should not be dismissed. The mobilization to prevent and stop peer victimization and bullying should expand to encompass sibling aggression as well.