Abstract: An improved noninvasive, electrooptical sensor probe is suitable for removable adhesive attachment to the skin of a patient to measure light extinction during transillumination of the blood-perfused tissue beneath the skin. The probe has a flexible generally U-shaped, web-like support structure having spaced, opposed upper and lower inner surfaces. A light source such as an LED is mounted in the support structure and positioned to emit light from one of the inner surfaces. A photosensor is also mounted in the support structure and positioned on the other of the inner surfaces to detect light emitted by the light source. A U-shaped alignment member is disposed in the support structure and extends substantially from the light source to the photosensor to hold the probe into the desired shape for use and aid in aligning the LED and photosensor.