Refractivity profiles from CHAMP GPS radio occultation measurements have been assimilated into the Met Office numerical weather prediction (NWP) system. A forecast impact experiment was run using 16 days of CHAMP data from May/June 2001, in addition to conventional and satellite observations which are used in operational forecasts. Although typically only 160 CHAMP profiles are assimilated daily, it is demonstrated that they reduce NWP temperature analysis and forecast mean and root mean square (RMS) differences against radiosondes in the upper troposphere and lower stratosphere. A significant impact is found in the Southern Hemisphere where 24 to 96 hour forecast 250 hPa temperature RMS differences are reduced by ~0.1 K. No improvement in humidity forecasts is found because refractivity values below 4 km are not used in the experiment. These results are encouraging and would support the case for using CHAMP measurements in NWP if they were available in near real time.