WATER vapour in the atmosphere is the key trace gas controlling weather and climate, and plays a central role in atmospheric chemistry, influencing the heterogeneous chemical reactions that destroy stratospheric ozone, Although in the upper troposphere and lower stratosphere the radiative(1) and chemical(2) effects of water vapour are large, there are few measurements of water-vapour concentration(3-10) and its long-term variation(11-13) in this region, Here we present is set of water-vapour profiles for altitudes from 9 to 27 km, obtained at Boulder, Colorado, during 1981-94, which show a significant increase in water-vapour concentration in the lower stratosphere over this time. The increase is larger, at least below about 20-25 km, than might be expected from the stratospheric oxidation of increasing concentrations of atmospheric methane(14,15). The additional increase in water vapour may be linked to other climate variations, such as the observed global temperature rise in recent decades(16).