A detailed analysis has been made of Stratospheric Aerosol and Gas Experiment I and II aerosol extinction data for the upper troposphere (6-km altitude to the seasonally averaged tropopause) taken between 1979 and 1998. An improved method of separation of the volcanic and surface-derived components of the aerosol optical depth has been used. The mean extinction, at a wavelength of 1.02 μm, of the nonvolcanic component of the upper tropospheric aerosol is found to increase from approximately 1¿10-4 km-1 at 70 ¿S to about 7 times that value at 70 ¿N. Maximum downward transfer of volcanic material into the upper troposphere is observed to take place in local spring in each hemisphere, occurring at a latitude of 70 ¿S or greater in the southern hemisphere and at about 50 ¿N in the northern hemisphere. The almost 20-year data sequence (1979--1981, 1984--1991, 1994--1998) has been examined for evidence of any long-term trends in the aerosol optical depth of the upper troposphere. It is unlikely that any change in the upper tropospheric 1-μm aerosol optical depth greater than 1% per year has taken place when averaged over either hemisphere. ¿ 1998 American Geophysical Union