Purpose/Objective: Existing evidence suggests that the relationship between adolescent behavior problems following traumatic brain injury (TBI) and injury-related family burden may be bidirectional, with increased child behavior problems contributing to greater family burden over the first year postinjury and vice versa. We extended existing evidence by prospectively examining reciprocal influences between parent-adolescent interactions and adolescent behavior problems over the initial 2 years postinjury. Research Method/Design: Participants included 117 adolescents ages 12-17 with moderate-to-severe TBIs and their parents who participated in a randomized controlled trial. At baseline, adolescent-parent dyads completed a videotaped problem-solving task that yielded composites of negativity, effective communication, and warmth. Parents also completed a structured interview and problem checklists. Families repeated assessments at 6, 12, and 18 months postbaseline. Bidirectional associations between observed parenting behavior and adolescent behavior were examined with autoregressive cross-lagged panel analyses collapsed across the trial arms. Results: One cross-lagged model was significant, with higher observed effective communication predicting fewer externalizing behavior problems at subsequent time points, and fewer externalizing behavior problems predicting more effective communication. Other models indicated that effects were unidirectional, with observed parenting behaviors predicting externalizing behavior and everyday functioning. Conclusions/Implications: The bidirectional relationship between effective communication and adolescent externalizing behavior suggests a transactional model in pediatric TBI in which effective communication leads to reduced adolescent behavior problems and reduced behavior problems lead to improved communication over time. Findings yielded stronger evidence for parenting effects as compared to adolescent behavior effects, providing further support for clinical interventions targeting parent-adolescent interactions following TBI. (PsycInfo Database Record (c) 2021 APA, all rights reserved).