Trends in O3 calculated from solar backscattered ultraviolet (SBUV) observations near 1 mbar are more negative at high latitudes in the southern hemisphere than in the northern hemisphere <Hood et al., 1993>. A mechanism is presented that produces an interhemispheric O3 trend asymmetry similar to the observed asymmetry in the Goddard Space Flight Center dynamically interactive zonal mean model. Upper Atmosphere Research Satellite (UARS) data are then examined for evidence that the atmospheric trend asymmetry is produced by a similar mechanism. The model O3 trend asymmetry is mainly due to interhemispheric differences in odd chlorine (Cly) partitioning. The asymmetry in Cly partitioning is caused primarily by lower amounts of CH4 and NO in the southern hemisphere than the northern hemisphere high latitudes, due to differences in the dynamical behavior of the model hemispheres. Symmetric increases in Cly are accompanied by a southern hemisphere increase in ClO that is larger than in the northern hemisphere. Concentrations of CH4 and N2O retrieved by the cryogenic limb array etalon spectrometer aboard UARS during 1992 are lower in the southern hemisphere fall and winter seasons at high latitudes in the upper stratosphere than in the northern hemisphere, favoring higher southern hemisphere ClO values. However, observations of ClO by the microwave limb sounder on UARS do not show consistently higher values in the southern hemisphere compared with the northern hemisphere in 1992, 1993, or 1994. The UARS data therefore do not confirm that a mechanism similar to the model mechanism occurs in the real atmosphere and is the cause of the SBUV O3 trend asymmetry. ¿ 1998 American Geophysical Union