Commercially available skin surface PCO2 sensors, when properly maintained, calibrated, and applied, report arterial PCO2 over a wide range of values and in virtually all clinical conditions to an accuracy of +/- 3 torr. Inappropriate mathematical treatment of in vivo skin surface-arterial PCO2 comparisons has led to controversy regarding the precise relationship between these variables. The proper method of calibration involves applying a temperature correction factor of 4.5%/degrees C to the calibration gas setting, and subtracting 4 torr by offsetting zero. For analysis of accuracy, the resulting corrected values should be used to determine the mean and standard deviation of the skin surface:arterial PCO2 ratio. Tests of correlation as a function of PaCO2 require deliberate wide variation of PCO2 within each subject of a test group. Skin surface PCO2 monitors record blood gas tensions continuously and noninvasively, and they can be used to study cardiorespiratory function in normal subjects, in whom arterial blood sampling would be difficult to justify--two distinct advantages of the devices.