The Nellcor pulse oximeter was studied in 16 newborn infants. The transcutaneous oxygen saturation (TCSaO2) was compared to both fractional saturation and functional saturation. TCSaO2 was found to be closer to fractional SaO2. 95% prediction intervals were established for the estimation of arterial oxygen saturation (SaO2) from TCSaO2. At a pulse oximeter reading of 85% to 90%, SaO2 could be as low as 83% and as high as 98%.