The earth's mesospheric carbon monoxide was observed in absorption against the moon in early December 1979 at a wavelength of 1.3 mm and in early December 1980 at a wavelength of 2.6 mm. These wavelengths correspond to the respective rotational transitions of CO, J=1 → 2 and J=0 → 1. No significant change in the column density of CO above ~65 km is found between the 1979 and 1980 observations. Inversion of the spectra did not provide unique CO mixing ratio profiles for a direct quantitative comparison of December 1979 and 1980 profiles, due to limited signal-to-noise ratios. Comparison with other published spectra of mesospheric CO suggests a large seasonal variation (about a factor of 2--3) in the column density of CO above 65 km, with a maximum in winter and a minimum in summer.