Measurements of mesospheric water vapor, by ground-based microwave spectroscopy, have been made over the period March 1984 to the present at Pennsylvania State University (Penn State) (41 ¿N). In this study we address the measurements made within the period November 1984 to July 1987; the data for March--May 1984 have been presented previously (Tsou et al., 1988). This is by far the longest record of mesospheric water vapor measurements yet obtained. We discuss the fundamentals of the technique and provide an extensive error analysis. We show monthly mean water vapor profiles for the altitude range 65--80 km and establish both annual and interannual variability. The observations consistently indicate that the seasonal variation of water vapor in the upper mesosphere is dominated by an annual component with low mixing ratios in winter and high mixing ratios in summer. The measurements are shown to generally compare favorably, in both the magnitude and seasonal variation of the mixing ratios, with similar measurements made at the Jet Propulsion Laboratory (JPL) (34 ¿N). However, at 80 km the Penn State measurements are nearly 1 ppmv lower than JPL results in the winter and early spring, which may be indicative of a large meridional gradient (or zonal asymmetry) during those seasons. The Penn State measurements are also shown to be in good general agreement with infrared solar occultation measurements made with the Spacelab 1 Grille and Spacelab 3 ATMOS experiments. ¿ American Geophysical Union 1989