A prospective study of 200 patients was conducted to evaluate the use of pulse oximetry as an adjunct to clinical monitoring of critically ill patients transported by rotary-wing aircraft with non-pressurized cabins. Thirty-four subjects (17%) were found to have significant hemoglobin desaturation of less than 90%, as defined by pulse oximetry (SpO2). Data were recorded continuously for later review. Desaturation often was noted prior to alterations in vital signs or clinical appearance. In 32 of the 34 hypoxemic subjects (94%), therapeutic interventions corrected the low SpO2. The use of pulse oximetry permitted measures for cardiorespiratory support to be instituted and assessed more rapidly than otherwise would have been possible. The availability of a continuous record of SpO2 facilitated detailed review of case management. It is concluded that the use of pulse oximetry is a practical and valuable adjunct for monitoring critically ill patients transported by rotary-wing aircraft.