PROJECT SUMMARY/ABSTRACT Prior studies demonstrate that 25-50% of adolescents with mild traumatic brain injuries (mTBIs) have substantial mental health sequelae during their recoveries. Moreover, 31-78% of children with TBI who have new or worsening mental health concerns are not receiving appropriate mental health care, with racial, ethnic, and economic inequities existing in post-TBI management and outcomes. While a substantial number of children experience ongoing or worsening mental health concerns in the first three months after mTBI, there are no validated prognostic tools to assess risk of ongoing or worsening mental health concerns in these patients. Our objective is to develop and validate a clinical tool to predict mental health sequelae in adolescents after mTBI. We will conduct a multicenter, prospective observational study in six PECARN (Pediatric Emergency Care Applied Research Network) emergency departments (EDs). We will enroll a derivation cohort (n=1512) at four sites and a validation cohort (n=1080) at two sites. Patients will be enrolled in EDs and have follow-up evaluations at 1 to 2 weeks, 1 month, and 3 months after the index ED visit. The primary outcome is a composite self-report measure of new or worsening mental health sequelae (based on minimal clinically important difference) 1 to 3 months after mTBI as measured by the Generalized Anxiety Disorder-7 and the Patient Health Questionnaire-8. We will also measure unmet mental health care needs, defined as not receiving any mental or behavioral health care in patients with new or worsening anxiety or depression. We selected sites with large numbers of children with mTBI and a high proportion of children from diverse backgrounds. Our study has the potential to impact the health and wellbeing of injured children worldwide. The results will be immediately significant, affecting both clinical practice, guidelines, and policy. The study will be conducted at PECARN sites, with the PECARN data coordinating center, and a centralized mental health outcome core, leveraging existing resources and investigators with multicenter, pediatric emergency care clinical prediction tool development experience.