The Geoscience Laser Altimeter System (GLAS) on the Ice, Cloud, and land Elevation Satellite (ICESat) provides a globally-distributed data set well suited for evaluating the vertical accuracy of Shuttle Radar Topography Mission (SRTM) digital elevation models (DEMs). The horizontal error (2.4 ¿ 7.3 m) and vertical error (0.04 ¿ 0.13 m per degree of incidence angle) for the ICESat data used are small compared to those for SRTM. Using GLAS echo waveforms we document differences between the SRTM C-band phase center and the highest, centroid, and lowest elevations within ICESat laser footprints in the western United States. In areas of low relief and sparse tree cover, the mean and standard deviation of elevation differences between the ICESat centroid and SRTM are -0.60 ¿ 3.46 m. The differences are -5.61 ¿ 15.68 m in high relief, sparse tree cover areas, and -3.53 ¿ 8.04 m in flat areas with dense tree cover. The largest differences occur in rugged, densely-vegetated regions.