OBJECTIVE To examine the influence of definition and location (field, emergency department, or pediatric intensive care unit) of hypotension on outcome following severe pediatric traumatic brain injury. DESIGN Retrospective cohort study. SETTING Harborview Medical Center (level I pediatric trauma center), Seattle, WA, over a 5-yr period between 1998 and 2003. PATIENTS Ninety-three children <14 yrs of age with traumatic brain injury following injury, head Abbreviated Injury Score > or = 3, and pediatric intensive care unit admission Glasgow Coma Scale score <9 formed the analytic sample. Data sources included the Harborview Trauma Registry and hospital records. INTERVENTIONS None. MEASUREMENTS AND MAIN RESULTS The relationship between hypotension and outcome was examined comparing two definitions of hypotension: a) systolic blood pressure <5th percentile for age; and b) systolic blood pressure <90 mm Hg. Hospital discharge Glasgow Outcome Score <4 or disposition of either death or discharge to a skilled nursing facility was considered a poor outcome. Pediatric intensive care unit and hospital length of stay were also examined. Systolic blood pressure <5th percentile for age was more highly associated with poor hospital discharge Glasgow Outcome Score (p = .001), poor disposition (p = .02), pediatric intensive care unit length of stay (rate ratio 9.5; 95% confidence interval 6.7-12.3), and hospital length of stay (rate ratio 18.8; 95% confidence interval 14.0-23.5) than systolic blood pressure <90 mm Hg. Hypotension occurring in either the field or emergency department, but not in the pediatric intensive care unit, was associated with poor Glasgow Outcome Score (p = .008), poor disposition (p = .03), and hospital length of stay (rate ratio 18.7; 95% confidence interval 13.1-24.2). CONCLUSIONS Early hypotension, defined as systolic blood pressure <5th percentile for age in the field and/or emergency department, was a better predictor of poor outcome than delayed hypotension or the use of systolic blood pressure <90 mm Hg.