The 5-day planetary wave in the polar mesosphere and lower thermosphere has been investigated using meteor radars at Esrange (68 degrees N, 21 degrees E) in the Arctic and Rothera (68 degrees S, 68 degrees W) in the Antarctic. The measurements span the 9-year interval from October 1999 to December 2008 and the 4-year interval from February 2005 to December 2008, respectively. The height range covered is approximately 80-100 km. Horizontal wind variance within a wave period range of 4-7 days is used as a proxy for the activity of the 5-day wave. Strong wave activity is seen in winter and late summer. However, there is a high degree of interannual variability, and in some individual years wave activity is almost absent. The data are used to construct a representative climatology for the Arctic and Antarctic. The seasonal cycle of the 5-day wave is found to be very similar in both polar regions. Wave activity in winter is present across the entire height range observed. Wintertime wave variance can reach about 65 m(2)/s(2). The wave is largely absent around the equinoxes. Wave activity is also very strong in late summer, reaching about 75 m(2)/s(2), but occurs only for 1-2 months and is confined to heights above about 90 km and excluded from the stronger westward winds at lower heights. Summertime wave activity peaks at a height of about 95 km and decays rapidly above and below that height. During this summer maximum, the wave cannot have ascended to the mesosphere and lower thermosphere from below and so must have been generated in situ and/or ducted across the equator from the winter hemisphere. The seasonal cycle of the 5-day wave is remarkably similar to that observed for the 2-day wave at these latitudes.