Pulse oximeters measure the arterial oxygenation by determining the color of the blood between a light source and a photodetector. The light source consists of two light-emitting diodes (LEDs) that emit light at known wave lengths, 660 nm red light and 940 nm infrared light. The ratio of pulse-added red absorbance at 660 nm to pulse-added infrared absorbance at 940 nm is used to generate the oximeter's estimate of arterial saturation (SpO2). Pulse oximeters can determine the concentration of only two hemoglobins, reduced (HHb) and oxyhemoglobin (HbO2), so they can't distinguish the dyshemoglobins (methemoglobin, carboxyhemoglobin) which have light absorbances similar to that of HHb or HbO2. If the concentration of dyshemoglobin is above the normal range, pulse oximeters would give erroneous SpO2 readings. We experienced a case which showed a low SpO2 reading but had normal ABGA findings due to unsuspected methemoglobinemia.