The accuracy of three ear oximeters (Waters XP-350, Waters 0-1100, Hewlett-Packard 47201A) was evaluated on 52 subjects during acute progressive and steady-state hypoxia. Ear oximeter readings were compared with measured arterial saturation (SAO2) in arterial or "arterialized" capillary blood samples. Hewlett-Packard (HP) 47201A ear oximeter readings most closely approximated measured Sao2 (y=0.99x - 1.52, r=0.97), although both the HP 47201A and Waters 0-1100 oximeters provided similarly accurate estimates of SAO2 once the characteristics of the individual oximeters were known (standard error of estimate; 2.52 and 2.98 per cent SAO2, respectively). The HP 47201A ear oximeter was relatively insensitive to changes in ear-piece position and differences in skin pigmentation, and offered the added advantages of comfort, simplicity of operation, and stability of characteristics.