This study reports on the first attempt to use cloud boundary and optical depth retrievals from the GLAS lidar to assess the accuracy of cloud-top heights obtained with MODIS (onboard NASA-TERRA and AQUA) and SEVIRI (onboard METEOSAT-8). Over the period from 25 September to 18 November 2003, MODIS-GLAS coincidences were only found in the Polar Regions, whilst SEVIRI-GLAS coincidences were available for a larger range of latitudes. For both passive instruments, cloud-top heights were overestimated by about 300--400 m when clouds were low and opaque. For high and thin clouds, MODIS and SEVIRI cloud-top heights were underestimated, and the bias increased with decreasing optical depth. For optical depths below 0.3, the number of misdetections increased significantly and the bias in cloud-top height increased from 1 to 2 km. The bias was found to be larger when lower cloud layers were present.