INTRODUCTION: Studies comparing children with and without a traumatic brain injury (TBI) have shown that pediatric TBI is associated with difficulties in a large range of functional domains, including emotion and behavior (Catroppa et al., 2015), as well as with changes in brain morphometry (King et al., 2019). However, whether these differences already existed before the injury and constitute a risk-factor, or emerge as a consequence of the injury remains an unsolved question. The large population-based Adolescent Brain Cognitive Development (ABCD) Study (Casey et al., 2018) provides a rare opportunity to explore this question. OBJECTIVE: This pre-post design study aimed to examine longitudinal changes in mental health, and brain morphometry following pediatric mild TBI (mTBI). METHODES: The following baseline and 2-year follow-up data from the ABCD 4.0 curated data release were used: 1) The Parent Ohio State TBI Screen-Short Modified Report (Bogner et al., 2017) to retrospectively identify children with no-TBI (n = 6,780; baseline mean age = 9.91 years; 3,309 [49%] males) and children who sustained a mTBI (i.e., head or neck injury with loss of consciousness ≤ 30 min or memory loss) between baseline and a 2-year follow-up (n = 145; baseline mean age = 9.97 years; 86 [59%] males), 2) internalizing and externalizing problem t-scores (syndrome scales) and DSM-5-oriented scales from the Child Behavior Checklist (CBCL, Achenbach & Rescorla, 2000), and 3) right and left hemisphere total cortical volume and mean cortical thickness computed on T1-weighted images by the ABCD group. Scanner effects were controlled for using longitudinal-ComBat (Beer et al., 2020). Multigroup latent change score models were constructed with the lavaan 0.6-12 package (Rosseel, 2012) in R (version 4.1.2) to estimate individual change between baseline and follow-up for mental health (CBCL scores), cortical volume and cortical thickness. Group differences in 4 parameters of interest (mean of the baseline score, rate of change over time, and variances of the baseline and of the change) were tested using chi-square difference tests (Kievit et al., 2018). Sex and parental education were included as covariates. RESULTS: As compared to non-injured peers, children with mTBI show no different trajectories in internalizing and externalizing problems, but different trajectories in ADHD and anxiety disorder scores. While similar at baseline (pre-injury), ADHD and anxiety disorder scores increased in children with mTBI and decrease in their non-injured peers. Children with mTBI showed greater cortical volume and thickness at baseline compared to non-injured peers, but no differences in change. CONCLUSION: This pre-post study suggests an increased risk of anxiety disorder and ADHD as a consequence of a mTBI, rather than a pre-existing risk factors. Further explorations using additional brain metrics and timepoints (ABCD release 5.0-fall 2022) will be conducted to fully understand these findings.