We used an 0.018-inch guidewire with a pressure sensor to measure arterial transstenotic pressure gradients. Our aim was to evaluate the pressure-recording properties of this device in vivo.Stenoses in the common carotid artery of piglets were created with external cylindrical constrictors. Pressures and pressure gradients were measured with the pressure wire and compared with reference measurements and with a 3.1-French fluid-filled catheter.The averages of several pressure measurements were close to the reference, but there were individual deviations. Twenty-nine percent of zero-corrected systolic measurements deviated more than 5 mm Hg from the reference, and 10% deviated more than 10 mm Hg. Errors canceled out somewhat in gradient measurements.Although some measurement errors were found, the guidewire represents an important new concept for gradient measurements. The small diameter minimizes gradient augmentation caused by the measuring device lying across the stenosis. Further refinement would increase its usefulness.