In recent years, estimates of UV radiation at the Earth's surface have been obtained from satellite retrievals. These retrievals have been compared with ground-based measurements at a few sites, but so far there have been few comparisons from unpolluted sites. This study compares differences between satellite-derived estimates of UV and ground-based measurements at a clean air site in the Southern Hemisphere with those from more polluted sites in the Northern Hemisphere, to investigate the extent to which boundary layer extinctions are taken into account by frequently used satellite retrievals. It is found that the hemispheric differences inferred from the ground-based measurements are much larger than those derived from the satellite retrievals. Since international intercomparison campaigns have shown the ground-based spectroradiometers to be in agreement, it is concluded that differences in tropospheric extinctions (e.g., by ozone and aerosols) are not adequately taken into account in the satellite retrievals of UV radiation examined so far. ¿ 2001 American Geophysical Union