WHEN the possibility of anthropogenic ozone depletion was first identified, it was thought that it would occur primarily in the upper stratosphere, at altitudes near 42 km. It is now recognized that ozone losses due to heterogeneous reactions involving chlorine and bromine1 are greatest in the lower stratosphere, near 20 km (ref. 2), and this is the main cause of ozone depletion over polar latitudes3. Despite satellite observation of the upper stratosphere for nearly a decade4, the question of possible ozone depletion in this region has remained unresolved because of instrument degradation and incomplete monitoring5. Recent observations of ozone concentrations in the upper stratosphere have been made with the Shuttle Solar Backscatter Ultraviolet (SSBUV) spectrometer carried by the Space Shuttle. Here we combine the SSBUV data for October 1989 with measurements made in October 1980 by the similar SBUV instrument on NASA's Nimbus-7 satellite, to show that the ozone concentration near 45 km has decreased during this period by about 7 ± 2%. The trend is consistent with the predictions of a two-dimensional photochemical model. Although this contribution to total column ozone depletion is small, the changes may have implications for the radiative properties of the upper atmosphere.