This paper estimates global changes in ultraviolet radiation since preindustrial times, resulting from anthropogenic stratospheric and tropospheric ozone changes. A sophisticated radiative transfer scheme is employed to calculate changes in the diurnally integrated UVB (defined here as 280--320 nm) and erythemally weighted (EER) irradiance reaching the Earth's surface. Stratospheric ozone changes are obtained from combining total ozone mapping spectrometer satellite data with Dobson ground-based instrument data. Tropospheric ozone changes are estimated from the University of Oslo three-dimensional chemical transport model. These ozone trend data are combined in three ways to calculate the associated ultraviolet trend and estimate its uncertainty. We show that in low latitudes a significant (up to 5% for UVB and 9% for EER) decrease in surface UV may have occurred, due to tropospheric ozone increases. Large UV increases are found at high latitudes, with up to 60% increases in October and 20% increases in April. The possible effects of soot and sulphate aerosol changes on the UV are also briefly examined; increased aerosol amounts may have decreased surface UVB and EER by up to 7% locally and about 2% on a global average. ¿ 1998 American Geophysical Union