Aircraft exhaust may reduce the crystal size in natural cirrus. This work investigates the change in cloud radiative forcing from such a size reduction by assuming a constant ice water content. A 1-dim model with radiative properties that depend on the mean crystal size is used to compute the radiative transfer for an atmospheric column. The results show that the negative shortwave cloud forcing is enhanced with smaller crystals as they mainly increase the reflectivity of clouds. The change in the longwave cloud forcing is always positive although its magnitude depends strongly on the ice water path. The weighted sum of SW and LW cloud forcings depends on the mean crystal size, surface albedo and ice water content. It appears that there is a range of diameters between 15 and 25 &mgr;m where the response to a reduction in crystal size is fairly insensitive. Below and above this range the change is negative or positive, respectively. In regions of dense airtraffic the magnitude of the change in cloud forcing could be on the order of 0.3 W m-2 under the assumption of a 20% decrease of the mean crystal size from about 30 &mgr;m to 24 &mgr;m. Aircraft exhaust thus has the potential to affect the climate but the results should be taken with caution as they are based on parameterized optical properties for cirrus clouds. ¿ 1998 American Geophysical Union