As triage and resuscitation protocols evolve, it is critical to determine the major extracranial variables influencing outcome in the setting of severe head injury. We prospectively studied the outcome from severe head injury (GCS score < or = 8) in 717 cases in the Traumatic Coma Data Bank. We investigated the impact on outcome of hypotension (SBP < 90 mm Hg) and hypoxia (Pao2 < or = 60 mm Hg or apnea or cyanosis in the field) as secondary brain insults, occurring from injury through resuscitation. Hypoxia and hypotension were independently associated with significant increases in morbidity and mortality from severe head injury. Hypotension was profoundly detrimental, occurring in 34.6% of these patients and associated with a 150% increase in mortality. The increased morbidity and mortality related to severe trauma to an extracranial organ system appeared primarily attributable to associated hypotension. Improvements in trauma care delivery over the past decade have not markedly altered the adverse influence of hypotension. Hypoxia and hypotension are common and detrimental secondary brain insults. Hypotension, particularly, is a major determinant of outcome from severe head injury. Resuscitation protocols for brain injured patients should assiduously avoid hypovolemic shock on an absolute basis.