Hypoxia and hypotension are potent causes of avoidable secondary brain damage after severe head injury. These systemic insults were studied in three cohorts of patients (600 in all) transferred in coma from general hospitals to a regional neurosurgical unit between 1979 and 1990. The incidence of hypoxia and hypotension on arrival at the neurosurgical unit fell from 30% in 1979-80 to 12% in 1989-90, even though the proportion of patients arriving within three hours of injury rose from 33% to 52%. Hypoxia was associated with airway obstruction, and hypotension with unsuspected or undertreated multiple injuries. Whatever the patient's age, CT scan findings, or depth of coma, hypoxia and hypotension has independent and additive adverse effects on outcome. The improvements between 1979 and 1990 are attributed to better airway care, especially the increased use of intubation and mechanical ventilation during transfer, and to greater appreciation of how relatively simple measures can reduce the potential hazards of ambulance transfer.