Although pulse oximeters are increasingly used to monitor patients on medical and surgical wards, staff seem to receive no specific training in the operation of these devices. We investigated the knowledge of junior doctors and nursing staff about pulse oximetry as used on seriously ill patients in a District General Hospital in Exeter, UK. 30 medical or surgical preregistration house officers or senior house officers and 30 staff nurses answered a structured questionnaire. Questions were asked about the theory behind pulse oximetry, factors affecting readings, "normal" values in various patients, values in hypothetical clinical situations, and what training subjects had received. Responses were scored against standard answers from reference texts. 97% of doctors and nurses did not understand how a pulse oximeter worked and were confused about factors influencing readings. Respondents gave a wide range of acceptable saturation values (eg, 90-100% for a fit adult), thus demonstrating poor understanding of physiological principles. There were serious errors made in evaluating saturation readings in hypothetical clinical situations. Only 1 doctor had received formal training in the use of pulse oximetry. Our survey revealed that junior doctors and staff nurses were untrained in pulse oximetry, lacked knowledge of basic principles, and made serious errors in interpretation of readings. Training is needed for staff who use pulse oximeters.