To determine the number of children with severe brain injury due to closed head injury or hypoxic-ischemic encephalopathy as a proportion of all admissions of children <3 yrs of age in the regional pediatric intensive care unit; to determine the outcome of these children at >6 mos postinjury; and to explore the relationship of outcome measures to predictors of outcome obtained within the first 24 hrs after brain injury. Prospective, descriptive outcome study of an inception cohort. Neonatal and Infant Follow-up Clinic, Glenrose Rehabilitation Hospital, Edmonton, Canada. Of a cohort of 53 children of <3 yrs of age (4% of pediatric intensive care unit admissions, 1995-1998) admitted for severe acquired brain injury (Glasgow Coma Score, <or=8), 12 (23%) died, three (6%) were lost to follow-up, and 38 (71%) survived and received multidisciplinary outcome assessments. Of 50 children with known outcome, 23 (46%) had a good recovery on the Glasgow Outcome Scale; however, only eight (16%) of these had average or above scores on both the mental and motor developmental indices of the Bayley Scales of Infant Development-II at 18-36 mos of age. Acute injury predictors of adverse outcome were apnea at scene, bolus epinephrine given for resuscitation, pupils nonreactive, arterial pH <or=7.2, and Glasgow Coma Score of 3. For 38 survivors, sensitivity and specificity from predictors for the Glasgow Outcome Scale were 73% and 83% respectively; for mental scores, 86% and 93%; for motor scores, 75% and 89%. Of children of <3 yrs of age identified by a Glasgow Coma Score of <or=8 after acute brain injury, >80% have adverse outcome of death, disability, or mental or motor developmental scores below average. A Glasgow Outcome Scale of 5 overestimates good recovery. Universal registry and follow-up of these children are needed.