Within the MEDCAPHOT-TRACE program of the European Community for studying the photochem. air pollution over the Great Athens Area, we report on 3-D lidar measurements of tropospheric ozone. After a general presentation of the DIAL technique, the exptl. layout of our system which is equipped with a unique capability for real-time anal. of each individual optical backscattered signal is briefly described and its performance is assessed. The temporal evolution of the ozone concn. up to the top of the boundary layer is then followed over a few days. In particular, the first day of the photochem. smog episode is presented and commented.