The lifetime of aircraft exhaust is estimated using multiyear diabatic trajectory integrations using United Kingdom Meteorological Office and Goddard Space Flight Center (GSFC) Data Assimilation Office winds. Instantaneous January trajectory parcel releases in the midlatitude stratosphere at 19, 16, 14, 13, and 11 km are compared. The first two cases correspond to the proposed flight altitudes of high-speed supersonic aircraft. The last two cases correspond to subsonic flight altitudes. Parcels descending below 250 hPa are eliminated as would occur when aircraft effluent reaches the troposphere. After a transient adjustment period the total number of parcels in most experiments decays exponentially with a decay period (lifetime) of about 1 year. This is the lifetime of the longest-lived eigenmode as discussed by Prather <1996>. Our 1-year lifetime is shorter than the 1.5 year GSFC two-dimensional (2-D) model lifetime for the same release, but in good agreement with the 1.13 year GSFC 3-D model lifetime. We also find that the effluent lifetime is slightly longer in the southern hemisphere midlatitude stratosphere than in the northern. Both the 13 and 11 km releases are mostly within the stratospheric middle world and rapidly leave the stratosphere. We conclude that there will be a small impact of subsonic aircraft exhaust on the larger stratosphere. Finally, we find that our overall lifetime results are fairly sensitive to the lower stratospheric heating rates. ¿ 1998 American Geophysical Union