Recent improvements in the noninvasive measurement of oxygen saturation by pulse oximetry (SapO2) have resulted in the development of easily used bedside monitors. We compared the performance of the Biox 3700 and Nellcor N100E models against arterial saturation (SaO2) measured on a Radiometer OSM2 hemoximeter and against transcutaneous SO2 (StcO2) derived from transcutaneous oxygen tensions (PtcO2) (632, Roche Kontron). Twenty-four hemodynamically stable patients were studied, all with indwelling arterial lines, aged between 1 month and 13 yr. Mean differences +/- SD between SapO2 and SaO2 were -0.77 +/- 3.23 for the Nellcor and -2.90 +/- 2.19 for the Biox. The Biox machine generally read lower than the Nellcor with a mean difference of -1.63 +/- 2.65. SaO2 derived from PtcO2 (StcO2) did not differ significantly (-0.27 +/- 2.12) from SaO2. In hemodynamically stable infants and children, StcO2 and SapO2 have similar accuracy limits when compared with each other and with SaO2.