The conjunctival oxygen tension (CjO2) sensor is a non-invasive, continuous index of oxygen delivery in the haemodynamically unstable patient. Human and animal studies have indicated that CjO2 reflects cerebral blood flow and oxygenation. Simple insertion, rapid stabilization and reaction time less than 60 s allow use in the initial stages of cardiopulmonary resuscitation (CPR) where invasive monitoring is often impracticable. CjO2 was monitored to assess cerebral oxygenation during CPR of 15 patients in cardiac arrest in the accident and emergency department (A&E). Patients who arrested out of hospital with delay to advanced cardiac life support and died had CjO2 less than 20 mmHg (normal CjO2 50-60 mmHg) on arrival in A&E. CPR with closed chest cardiac massage (closed CPR) produced no improvement in CjO2. Patients who arrested in ventricular fibrillation (VF) in A&E, and survived with no neurological deficit had CjO2 greater than 20 mmHg during CPR. However, further episodes of VF produced an immediate fall in CjO2 which continued, despite closed CPR, until restoration of spontaneous cardiac output (RSCO) determined by a palpable carotid pulse. In survivors with delay from arrest to CPR the rise in CjO2 with RSCO did not occur for up to 10 min. This study suggests that closed CPR has no value in maintaining or improving cerebral oxygenation during cardiac arrest. Further studies are required to determine the precise relationship of CjO2 to cerebral blood flow and oxygenation during CPR using open and closed techniques of cardiac massage. Open chest cardiac massage (open CPR) has been shown to produce near normal cerebral perfusion and if patients are to survive prolonged resuscitation neurologically intact guidelines for open CPR must be reviewed.