Stratospheric water vapor concentrations measured at two midlatitude locations in the northern hemisphere show water vapor amounts have increased at a rate of 1--1.5% yr-1 (0.05--0.07 ppmv yr-1) for the past 35 years. At Washington, D.C., measurements were made from 1964--1976, and at Boulder, Colorado, observations began in 1980 and continue to the present. While these two data sets do not comprise a single time series, they individually show increases over their respective measurement periods. At Boulder the trends do not show strong seasonal differences; significant increases are found throughout the year in the altitude range 16--28 km. In winter these trends are significant down to about 13 km. ¿ 2000 American Geophysical Union