Both in situ measurements and satellite remote sensing data show that water vapor in the extratropical lowermost stratosphere has a pronounced seasonal cycle, with a maximum in summer. In this paper we show that the seasonal cycles derived from satellite measurements by the Stratospheric Aerosol and Gas Experiment (SAGE) II and the Microwave Limb Sounder (MLS) and ER-2 aircraft measurements are in reasonable agreement when compared using potential vorticity (PV) and potential temperature (&thgr;) binned monthly averages. The annual means and amplitudes of the water vapor seasonal cycle are derived from the SAGE II data for 320--360 K and 2--7 PV units (covering latitudes near and poleward of the tropopause). The high moisture content and the summer maxima indicate that the effect of transport from the troposphere across the extratropical tropopause can be seen in nearly the entire range examined and is more significant in the lower PV-&thgr; bins. We further investigate mechanisms that control the amount and seasonal cycle of water vapor in the lowermost stratosphere. One important issue is whether local temperature near the extratropical tropopause limits isentropic transport of water vapor into the lowermost stratosphere, similar to the freeze-out of water vapor crossing the tropical tropopause. A quantitative comparison of saturation vapor mixing ratios (derived from National Centers for Environmental Prediction temperature analyses) with the water vapor measurements shows that the amount of water vapor near the extratropical tropopause is substantially lower than that given by saturation over ice. This demonstrates that local temperature effects do not set the upper limit of water vapor in this region. Analyses using ER-2 measurements confirm that although saturation and supersaturation do occur, on average, air near the extratropical tropopause is undersaturated. The seasonal cycle of the fraction of the overworld air in the lowermost stratosphere is inferred using the water vapor and saturation vapor mixing ratio climatology. ¿ 2000 American Geophysical Union