The present study compares the outcome of adult and pediatric patients with severe diffuse brain injury, and analyzes factors affecting the prognosis related to age difference. Of 912 patients admitted with head injury during the past three years, 223(1.7%) patients were identified as severe diffuse brain injury. Among the 223 patients. 100 patients were 15 years of age or less as the pediatric group. The mortality rate for pediatric and adult group was 39.0% and 48.8%, respectively. The common factors affecting poor prognosis for both groups were Glasgow Coma Scale(GCS) of 5 or less, papillary abnormality, hypoxia(PaO2<60 mmHg), the presence of skull fracture(basilar skull fracture in children, and basilar or vault skull fracture in adult group), diffuse brain swelling, subarachnoid hemorrhage, intraventricular hemorrhage, and thin subdural hematoma. The factors affecting prognostic difference between the adult and pediatric group with diffuse brain injury were the presence of vault skull fracture(p<0.01) and diffuse brain swelling(p<0.05). In patients associated with vault skull fracture or diffuse brain swelling, adult group had a significantly poorer outcome than child group.