The radiative forcing of the Earth's climate system due to tropical tropospheric ozone is estimated using ozonesonde profiles and maps of the tropospheric ozone column derived from satellite data. The forcing is estimated using several different techniques in order to place bounds on its likely magnitude and to elucidate the role of biomass burning in producing the observed forcing. The results suggest that a widespread radiative forcing of at least 0.5 to 1 W m-2 exists over large areas in the tropics for much of the year. This radiative forcing is comparable in magnitude, but opposite in sign, to estimates of the aerosol forcing from tropical biomass burning. However, the burning contribution to ozone forcing is present over a larger geographic area than the aerosol forcing. The majority of the burning and thus these radiative forcings have likely been present only in the past century. These enhancements in tropical ozone are also estimated to be responsible for a radiative forcing of between 0.1 and 0.4 W m-2 when globally averaged. Comparison of these regional and global forcings due to tropical ozone changes to the estimates of forcing due to carbon dioxide and other trace gas increases since preindustrial times (about 2.45 W m-2) suggests that the effects of tropical ozone changes could be significant for evaluation of both regional and global anthropogenic forcing of climate.¿ 1997 American Geophysical Union