Forward scattering in cirrus clouds can significantly affect backscatter lidar signal. Its contribution can be estimated numerically knowing the size and the shape of the crystals. Contemporary airborne and space-borne lidar measurements collected during the European Lidar In-space Technology Experiment (E-LITE) field campaign (in the framework of the NASA/LITE mission) are used to show that forward scattering reduces the optical depth derived from space-borne measurements by at least a factor of 2 with respect to the true value. This reduction factor is discussed in terms of crystal size and shape (i.e. forward scattering, and measurement configuration). It is shown that cirrus backscatter-to-extinction ratios derived from airborne and space-borne lidar measurements can be used to illustrate the sensitivity of multiple scattering effects the ice crystal size and shape. ¿ 1999 American Geophysical Union