We present middle atmospheric water vapor measurements made at 22 GHz using two Naval Research Laboratory Water Vapor Millimeter-wave Spectrometers (WVMS2 and WVMS3). We include measurements from an intercomparison campaign at Table Mountain, California (34.4 ¿N, 242.3 ¿E), and measurements obtained since March 1996 from Table Mountain and from Mauna Loa, Hawaii (19.5 ¿N, 204.4 ¿E). The mesospheric data from Mauna Loa show both a mixing ratio peak at a higher altitude and smaller seasonal variations than those from the WVMS instrument at Table Mountain. These differences are qualitatively consistent with the changes in mixing ratio and the increase in mesospheric variability expected with decreasing latitude. The latitudinal variation in the mixing ratio profile observed in the WVMS data is very similar to that observed by the Halogen Occultation Experiment (HALOE) in the upper stratosphere and lower mesosphere. The WVMS measurements generally show higher water vapor mixing ratios than HALOE does in the upper stratosphere and lower mesosphere but smaller mixing ratios in the upper mesosphere. The winter of 1997-1998 shows an unusually large decrease in mesospheric water vapor, a result that is consistent with measurements from HALOE throughout the Northern Hemisphere for this winter. There appears to be a slight overall decrease in water vapor for all seasons over the period of the WVMS observations from Mauna Loa. The relatively small seasonal variations, combined with the small amount of tropospheric water vapor above Mauna Loa, make this an ideal site for the monitoring of multiyear trends in water vapor. ¿ 1999 American Geophysical Union