To prevent secondary cerebral ischemia in comatose patients it would be of great importance to assess cerebral blood flow. Recently monitoring of the jugular venous oxygen saturation (SJVO2) has been shown to continuously evaluate cerebral oxygenation and to estimate cerebral blood flow. While most of these studies have dealt with severely head injured patients, we investigated cerebral oxygenation in 50 comatose patients due to an intracerebral haematoma (n = 14), subarachnoid haemorrhage (n = 12) and severe head injury (n = 24). In these groups of patients, the reaction of SJVO2 to hyperventilation and to lowering of blood pressure was studied. Moderate hyperventilation from 35 to 28 mmHg resulted in a significant decrease of SJVO2 in all groups. A critical SJVO2 between 50 and 55% was found in one half of the patients studied, a pathological SJVO2 below 50% was seen in 23% of the cases. Lowering of arterial blood pressure within the limits of autoregulation resulted in decreases of SJVO2 in patients with intracerebral haematomas only. 55% of these patients showed signs of insufficient cerebral oxygenation. Furthermore the frequency of spontaneous desaturation episodes was studied retrospectively and comparison made between the different groups. These episodes were found more frequently in patients with intracerebral haematomas compared to patients with severe head injury. In conclusion, monitoring of jugular venous oxygen saturation is a valuable tool for detecting and treating insufficient cerebral oxygenation in comatose patients following intracerebral haemorrhage, subarachnoid haemorrhage and severe head injury.