The response of the middle atmosphere to an increase in humidity arising from a possible future increase in CH4 is examined in a general circulation model with interactive H2O and O3. A chemical parameterization allows the middle-atmospheric H2O change to evolve naturally from an imposed change in tropospheric CH4. First, a simulation of the year 2060 using postulated loadings of the radiatively active gases is compared with a control simulation of the present-day atmosphere. Then, the particular contribution of the CH4 (and hence H2O) change to the observed difference is isolated by repeating the 2060 simulation without the projected CH4 change. Under the Intergovernmental Panel on Climate Change Special Report on Emission Scenarios (SRES) B2 scenario, the middle atmosphere in 2060 cools by up to ~5 K relative to 1995, with the CH4-derived increase in H2O accounting for ~10% of the change. The cooling is accompanied by a strengthened general circulation, intensified dynamic heating rates, and a reduction in the mean age of middle-atmospheric air. The component of the circulation change attributable solely to the H2O change differs somewhat from the net response: The H2O change causes a greater increase in the descent rate in the north than in the south, ages the stratospheric air, and has a distinct effect on age/N2O correlations. Around 20% of the increased prevalence of polar stratospheric clouds (PSCs) in 2060 is due to the microphysical effect of the extra H2O, with the remainder attributable to the reduced vortex temperatures. Although the PSC increase facilitates release of reactive chlorine, this positive impact on chemical O3 destruction is outweighed by the negative impact of the reduced total chlorine in 2060. Nonetheless, the H2O increase does make the 2060 Arctic O3 loss ~15% greater than it would otherwise be.