A chemical box model constrained by Upper Atmosphere Research Satellite (UARS) measurements of CH4, H2O, NOx, and temperature in 1992--1994 has been used in an attempt to simulate ozone trend measurements from the Stratospheric Aerosol and Gas Experiment (SAGE) at 43 km, 45¿ latitude and their hemispheric differences. The model is successful in simulating the seasonal cycle in ozone mixing ratios observed by the Microwave Limb Sounder (MLS) and the Solar Backscattered Ultraviolet (SBUV) instrument including the 15% larger values in the Southern Hemisphere at the wintertime ozone maximum. The model indicates that this hemispheric asymmetry is associated with hemispheric temperature differences of approximately 10¿K; in contrast, hemispheric differences in methane and the resulting effects on ClOx make only a small contribution to this ozone asymmetry. The model predicts that, based on observed Cly increases, the ozone decrease in the Southern Hemisphere should have exceeded that in the Northern Hemisphere by approximately 1%/decade. This is consistent with an observed ozone hemispheric trend difference of 1.7 ¿ 2.1%/decade (1σ) for 1979--1997 and -0.8 ¿ 2.8%/decade for 1985--1997 from SAGE I and SAGE II (version 6.1) measurements. These trend differences are caused by hemispheric differences in temperature in early winter and in CH4 in late winter. The model, based on changes in Cly but not in other specified parameters, calculates a larger downtrend of ozone than is observed by SAGE II in 1985--1997 by approximately 3.1 ¿ 1.4%/decade (1σ), but it qualitatively simulates the observed ozone trend variations over the period 1979--1997. The amplitude of the model calculated downtrend would be reduced by approximately 1%/decade for a methane increase of 1%/yr. Observed temperature decreases of approximately 0.1¿K/yr would have decreased the magnitudes of the calculated ozone downtrends on pressure surfaces by approximately 1%/decade but could have affected SAGE trends by a small amount in the opposite direction because of associated geopotential height changes.