Prompt recognition and treatment of hypoxia is an important part of management in the accident and emergency (A & E) department. Until recently the only reliable method of detecting hypoxia was by estimation of the arterial blood gases (ABG). Continuous monitoring of the arterial oxygen saturation (Sao2) is possible using an infra-red pulse oximeter. This study assessed the usefulness of this instrument in the A&E setting. The Sao2 was measured in 50 patients using a pulse oximeter. In 15 patients simultaneous ABG estimations were obtained. The Sao2 correlated closely with calculated values for Sao2. The use of the oximeter identified 21 patients (42%) with clinically unsuspected hypoxia. The pulse oximeter proved simple to use, accurate and a useful addition to our resuscitation equipment.