Atmospheric water vapor data derived from the special sensor microwave/imager (SSM/I) are used to make time-coincident, wet tropospheric range corrections to Geosat altimeter data in the northeast Pacific. The original and corrected sea surface height residuals along numerous tracks are examined to determine the impact of water vapor on the altimeter signal. Mesoscale feature analyses of corrected and uncorrected altimeter data are used to assess the impact of water vapor path lengthening in areas of low sea surface height variability. Results indicate that the horizontal spatial variations in the water vapor height corrections are frequently similar to true oceanographic gradients. Interpretation of altimeter data is affected in several ways. The unaccounted-for presence of atmospheric water vapor may mimic or mask the true ocean features, and even small changes in the water vapor over short spatial scales can enhance a partially obscured feature. Analyses of all Geosat tracks crossing the area of interest in September 1987 clearly illustrate that water vapor frequently contaminates the ocean topography measurements, making the water vapor adjustment critical before the altimeter data can be successfully used to locate and identify mesoscale ocean features. Furthermore, the SSM/I and Geosat data must be closely matched in both space and time, a difficult task since it takes 3.5 days to obtain global SSM/I coverage with one operational sensor. To optimize the mesoscale oceanographic application of altimeter data, a borne-sighted radiometer should be included on all altimeter spaceborne platforms.