BACKGROUND Physicians caring for newborns and infants residing at or traveling to moderate altitude have little information available about the normal range for arterial oxygen saturation (SaO2) measured by pulse oximetry. To aid clinicians in making rational decisions about the oxygen status of children at moderate altitude, we measured SaO2 in newborns and infants who came to two family practice offices located at an altitude of 2800 meters (9000 feet) to obtain normal values for both well-child and illness visits. METHODS SaO2 measured by pulse oximetry was recorded for children younger than 2 years seen consecutively in a family practice clinic for care for any reason. The children all resided at an altitude of 2800 m (9000 ft). RESULTS The mean SaO2 for healthy awake infants was 91.7 percent, significantly lower than the reported normal ranges for either sea level or Denver. Saturation levels in infants with minor acute illnesses did not differ from saturation levels in healthy infants, while infants with lower respiratory tract infections had significantly lower SaO2 measurements. CONCLUSIONS SaO2 levels are significantly lower in newborns and infants living at moderate altitude. Measurement of SaO2 at moderate altitude can be helpful in the care of both healthy and ill newborns or infants.