The present study tested the hypothesis that acute posttraumatic hyperarousal would lead to the development of emotional numbing (EN) symptoms in a pediatric injury population. Eighty-two youths aged 8–18 years were recruited from the emergency department of a Midwestern children's hospital. Heart rate was recorded from emergency medical services reports and a 12-hour urine collection was initiated upon admission. Six weeks and 6 months later, depression and PTSD symptoms were assessed. Initial heart rate and urinary cortisol levels predicted 6-week and 6-month EN after controlling for concurrent depression, avoidance, and reexperiencing symptoms and 6-week hyperarousal symptoms. These findings provide empirical support for prior hypotheses concerning the development of PTSD symptoms over time.