A middle ultraviolet imager observed the vertical structure of polar mesospheric clouds (PMCs) in the northern and southern polar regions during the austral summer of 1997--1998 and the arctic summer of 1999. During 23 transpolar passes, the imager obtained over 15,000 images at latitudes poleward of 55¿. The accuracy and stability of the satellite platform and the large database permit the first statistical investigation of the small scale (~1 km) altitude structure of PMCs on a transpolar scale. During a satellite pass over either polar region, PMCs clustered at different altitudes between 80 and 85 km. Southern clouds had a mean altitude of 83.2¿1.4 km above the Earth ellipsoid, while northern clouds had a mean altitude of 82.6¿1.3 km, so southern and northern clouds had the same altitudes to within statistical variations. The mean PMC altitude was 82.7¿1.3 km for both hemispheres. In neither hemisphere did cloud altitudes exhibit systematic variation with latitude. ¿ 2001 American Geophysical Union