Subsyndromal posttraumatic stress among pediatric cancer survivors has been associated with negative physical and mental health outcomes. However, adolescent self-report and mother-proxy report of adolescents’ posttraumatic stress symptoms evidenced varying concordance depending on methodology. There was moderate concordance, particularly among younger respondents, when total posttraumatic stress symptoms were viewed continuously and low-moderate concordance when viewed categorically; moderate-strong concordance for only one posttraumatic stress disorder symptom cluster; low-moderate agreement for high-frequency items; and no concordance for identifying caseness. Although a significant subset of pediatric cancer survivors experience posttraumatic stress, mothers and adolescents demonstrate limited symptom, categorical, and caseness agreement, potentially impacting adolescents’ healthcare service utilization.