A reflectance pulse oximeter (RPO) sensor has been designed to noninvasively measure arterial oxygen saturation (SaO2) from the forehead or cheek. It consists of 660- and 820-nm light emitting diodes (LEDs) and a photodiode chip mounted on a single substrate. Four LED chips for each wavelength were placed at equal distance around the photodiode to enhance the signal level as well as to minimize inhomogeneous effects of tissue. In animals, the RPO yielded better correlation than the transmission pulse oximeter (TPO) in the saturation range from 40-100%. The sensor was evaluated on 15 critically ill patients in the operating room. The RPO yielded accurate measure of SaO2 in comparison to the blood samples. It is concluded that the RPO can be a powerful alternative to the TPO in the operating room and intensive care unit, particularly in caring for critically ill patients, neonates, and infants.