We present new ground-based measurements of polar stratospheric and mesospheric CO, made with a millimeter-wave spectrometer at Thule, Greenland (76.5°N, 68.7°W). Almost daily measurements were made between 17 January and 4 March 2002 and again between 5 January and 22 February 2003. We stress here the retrieval and analysis of CO mixing ratios in the 50–80 km altitude range, though it can be monitored at lower altitudes as well. Since CO exhibits a strong positive latitude gradient from the summer to the winter pole, it is an excellent tracer for poleward transport from lower latitudes. Moreover, the mixing ratio of CO increases rapidly from ∼40 km to at least 100 km at midlatitudes, providing a good tracer for high-altitude vertical transport as well. Our profiles indicate that in winter near the poles the CO mixing ratio decreases above ∼70 km because of subsidence of air and minimal high-altitude photoproduction at high latitudes. Our data also show large variations in mixing ratio and vertical distribution, yielding a good picture of stratospheric and mesospheric dynamics-induced changes on a scale of hours to days. These observations verify that CO serves as an excellent tracer of vortex-related dynamics in the 30–80 km altitude range, where other information, particularly above ∼40 km, may be sparse, unreliable, or nonexistent. Our results are in general agreement with analyses of 1991–1992 Improved Stratospheric and Mesospheric Sounder (ISAMS) satellite data by Lopez-Valverde et al. [1993 , 1996] and by Allen et al. [1999 , 2000]. We show the contrast between CO over the summer pole and CO over the winter pole with the aid of trial observations made at the South Pole during the austral summer of 1999–2000. Our Thule data indicate that large concentrations of CO generally exist in winter just outside the vortex boundary. The large rapid variations in vertical profile that are found in our data in 2002 appear to be well correlated with vortex position in the lower stratosphere. In 2003 this correlation appeared to be much weaker, but early 2003 was also a period of vortex splitting in the Arctic on three occasions during our observation period, accompanied by generally more complex vortex dynamics.