The ozone profile calculated using a one-dimensional time-dependent photochemical model, with recommended absorption cross sections and reaction rate constants, is systematically less than Spacelab 3 (May 1985) Atmospheric Trace Molecule Spectroscopy (ATMOS) observations at sunset (&khgr;=90¿), 30 ¿N latitude, throughout the upper stratosphere and mesosphere. Model results are ~9% smaller than the nominal ATMOS O3 value at 40 km and 50% less at 76 km. Given the uncertainties associated with the ATMOS profile, the model ''deficit'' may be as large as 15% at 36 km, increasing to ~70% at 76 km. A similar model O3 deficit exists at sunrise, 47 ¿S latitude, in the stratosphere: nominal differences of ≤25%, increasing to ≤45% when the ATMOS uncertainties are considered. Model results closely approximate ATMOS observations in both the upper stratosphere and mesosphere upon enhancing odd-oxygen production by increasing the O2 Schumann-Runge band and Herzberg continuum cross section values used in the calculations. ¿ American Geophysical Union 1991