The continuous measurement of jugular venous oxygen saturation(SjvO2) with a fibroptic catheter is evaluated as a method of detecting cerebral ischemia after head injury. Fifty patients admitted to the hospital who were unconscious after severe head injuries had continuous and simultaneous monitoring of SjvO2, intracranial pressure, arterial oxygen saturation, arterial blood pressure. Whenever SjvO2 dropped to less than 50%, a standardized protocol was followed to confirm the validity of the desaturation and to elucidate its cause. A total of 72 episodes of jugular venous oxygen desaturation occurred in 45 patients, possibly due to intracranial hypertension in 39 episodes, arterial hypoxia in 13, combinations of the above in 9, systemic hypotension in 7, and anemia in 4. Two episodes of hyp-eremia, SjvO2 more than 90%, occurred in 2 patients with carotid-cavernous fistula. The incidence of jugular venous oxygen desaturation found in this study suggests that continuous monitoring of SjvO2 may be of clinical value in patients with head injury.