The outcome from severe head injury (GCS < or = 8 mmHg) was prospectively studied in patients from the Traumatic Coma Data Bank. We investigated the impact on outcome of hypotension (SBP < 90 mmHg) occurring from injury through resuscitation (early hypotension; N = 717) or in the Intensive Care Unit [ICU] (late hypotension; N = 493). Early hypotension occurred in 248 patients (34.6%) and was associated with a doubling of mortality (55% vs. 27%). If shock was present on admission, the mortality was 65%. These effects were independent of age, admission GCS motor score, presence of hypoxia, or associated severe extracranial trauma, suggesting that the influence of multiple system trauma in head injured patients is primarily due to associated hypotension. Late hypotension occurred in 156 of 493 patients (32%) and was the only hypotensive episode in 117 (24%). For 117 patients whose only hypotensive episode occurred in the ICU, 66% either died or were vegetative survivors, compared to 17% of patients who never suffered an hypotensive episode. Logistic regression modelling suggested that early and late shock were the most powerful independent predictors of mortality in this group of patients. These data demonstrate that hypotension is a common and devastating secondary brain insult in severe head injury patients, occurring not only during transport and resuscitation but also "right under our noses" in the ICU. We suggest that vigorous attention to eliminate or minimize such insults has the potential of markedly improving outcome from severe head injury.