We present water vapor measurements made by the Polar Ozone and Aerosol Measurement (POAM) III instrument since May 1998 in the upper troposphere and lowermost stratosphere. While POAM III is primarily a stratospheric instrument, many of the POAM III occultation measurements allow for the retrieval of water vapor in the upper troposphere. The Measurements of Ozone and Water Vapor by Airbus In-Service Aircraft (MOZAIC) instruments provide a large number of coincident measurements and thus offer the best opportunity to validate POAM measurements in the highly spatially variable regions of the upper troposphere-lowermost stratosphere, where the mixing ratios are much larger than those found throughout most of the stratosphere. The comparison shows that there is no statistically significant difference in the response of the two instruments to changes in water vapor and that in the regime where the MOZAIC measurements are thought to be most accurate, the water vapor mixing ratios measured by POAM are 10% higher. The POAM III Northern Hemisphere measurements are taken from 55¿ to 71¿ and show a qualitatively reasonable seasonal variation, with high mixing ratios in the upper troposphere in the summer and low mixing ratios in the winter. Comparisons of the seasonal variations of the POAM measurements with those from the upper tropospheric Microwave Limb Sounder (MLS) measurements from the early 1990s show qualitative similarities. The ~1 km vertical resolution of POAM measurements allows us to study in greater detail than other satellite instruments the complex variations in water vapor that occur in the upper troposphere and lowermost stratosphere. Among the interesting features observed is a rise in the level of the high-latitude hygropause from April through September.