Although researchers have found that child welfare placement disruptions are associated with elevated youth physical and mental health problems, the mechanisms that explain this association have not been previously studied. The present study built on a previous investigation of the physical and behavioral consequences of long-term permanent placement patterns among youth who participated in the Longitudinal Studies of Child Abuse and Neglect (LONGSCAN). The current investigation (n = 251) aimed to (a) report the early adolescent living situations of youth with different long-term placement patterns, and (b) to delineate the roles of adverse childhood experiences (ACEs) and post-traumatic stress (PTS) reactions in the association between unstable long-term placement patterns and physical and mental health problems during the transition to adolescence. Information about youth's living situations, ACEs, and physical and mental health was gathered prospectively from child protective services records and biannual caregiver and youth interviews when youth were 4-14 years old. The majority of youth remained with the same caregiver during early adolescence, but youth with chronically unstable permanent placement patterns continued to experience instability. Path analyses revealed that ACEs mediated the association between unstable placement patterns and elevated mental, but not physical, health problems during late childhood. Additionally, late childhood PTS mediated the association between unstable placement patterns and subsequent escalations in physical and mental health problems during the transition to adolescence. Findings highlight the importance of long-term permanency planning for youth who enter the child welfare system and emphasize the importance of trauma-focused assessment and intervention for these youth. (C) 2016 Elsevier Ltd. All rights reserved.