Data from two instruments onboard the National Aeronautics and Space Administration (NASA) ER-2 high-altitude aircraft have been utilized in the largest validation study to date in assessing the accuracy of the CO2-slicing cloud height algorithm. Infrared measurements of upwelling radiance from the MODIS (Moderate-Resolution Imaging Spectroradiometer) airborne simulator (MAS) were used to generate cloud top heights and then compared to those derived from the Cloud Lidar System (CLS), operating with dual polarization at 0.532 μm. The comparisons were performed for 10 flight days during the Subsonic Aircraft Contrail and Cloud Effects Special Study (SUCCESS) field experiment during April and May 1996 which included various single-layer and multilayer cloud conditions. Overall, the CO2-slicing method retrieved cloud heights to within ¿500 m and to within ¿1500 m of the lidar heights in 32 and 64% of the cases, respectively. From a simulation of cloud height errors as a function of various error sources in the CO2-slicing algorithm, it was concluded that the problem of multilayer clouds is secondary to that of proper specification of clear-sky radiances. ¿ 1999 American Geophysical Union