Infrared radiation from the earth may be the principal reason for the decay of LAGEOS' orbit. The radiation heats up the laser retroreflectors embedded in LAGEOS' aluminum surface. This creates a north-south temperature gradient on the satellite. The gradient in turn causes a force to be exerted on LAGEOS because of recoil from photons leaving its surface. The delayed heating of the retroreflectors due to their thermal model for the retroreflectors indicates that this thermal drag accounts for at least half the observed average along-track acceleration of -3.33¿10-12 ms-2 and perhaps much more. The contribution from the aluminum surface to this effect is negligible. Thermal drag, which is a variant of the Yarkovsky effect, can explain some; but not all, of the observed fluctuations in drag. In view of these results, charged particle drag may not be as important as previously believed for LAGEOS. ¿ American Geophysical Union 1987