Extensive analyses of ozone observations between 1978 and 1998 measured by Dobson Umkehr, Stratospheric Aerosol and Gas Experiment (SAGE) I and II, and Solar Backscattered Ultraviolet (SBUV) and (SBUV)/2 indicate continued significant ozone decline throughout the extratropical upper stratosphere from 30--45 km altitude. The maximum annual linear decline of -0.8¿0.2% yr-1(2&sgr;) occurs at 40 km and is well described in terms of a linear decline modulated by the 11-year solar variation. The minimum decline of -0.1¿0.1% yr-1(2&sgr;) occurs at 25 km in midlatitudes, with remarkable symmetry between the Northern and Southern Hemispheres at 40 km altitude. Midlatitude upper-stratospheric zonal trends exhibit significant seasonal variation (¿30% in the Northern Hemisphere, ¿40% in the Southern Hemisphere) with the most negative trends of - 1.2% yr-1 occurring in the winter. Significant seasonal trends of - 0.7 to - 0.9% yr-1 occur at 40 km in the tropics between April and September. Subjecting the statistical models used to calculate the ozone trends to intercomparison tests on a variety of common data sets yields results that indicate the standard deviation between trends estimated by 10 different statistical models is less than 0.1% yr-1 in the annual-mean trend for SAGE data and less than 0.2% yr-1 in the most demanding conditions (seasons with irregular, sparse data) <World Meteorological Organization (WMO), 1998>. These consistent trend results between statistical models together with extensive consistency between the independent measurement-system trend observations by Dobson Umkehr, SAGE I and II, and SBUV and SBUV/2 provide a high degree of confidence in the accuracy of the declining ozone amounts reported here. Additional details of ozone trend results from 1978 to 1996 (2 years shorter than reported here) along with lower-stratospheric and tropospheric ozone trends, extensive intercomparisons to assess relative instrument drifts, and retrieval algorithm details are given by WMO <1998>. ¿ 2000 American Geophysical Union