We present the first analysis of water vapor measurements made during a stratospheric warming using ground-based microwave spectroscopy. The measurements were made from the ALOMAR observatory (69 ¿N, 16 ¿E) between January 31 and February 10, 1998. The data show an increased mixing ratio between ≈45 and 70 km throughout the warming. The analysis uses temperature data obtained by lidar measurements from the same location. A comparison with results obtained using standard temperature data reveals the importance of 'good' temperature measurements for water vapor retrieval from remote sensing data during such events in order to separate temperature effects from atmospheric dynamics. ¿ 2000 American Geophysical Union