Ground-based lidar measurements of ozone were conducted at the Observatoire de Haute Provence (OHP) (44¿N, 6¿E, 700 m above sea level (asl)) to study ozone exchanges between the stratosphere and the troposphere. Three case studies are presented, corresponding to intensive lidar measurements obtained during two or three consecutive days within the range 5--11 km. Whenever it was possible, additional data such as backward/forward air mass trajectories, synoptic weather maps, and total ozone mapping spectrometer (TOMS) maps were also used to obtain a better understanding of the ozone increases and to infer the impact of stratospheric air mass at tropospheric level. For the three cases, elevated ozone content in the upper troposphere (>80 ppb at 6 km) corresponded to the passage of a frontal system above the OHP. An upper limit of 2.1033 ozone molecules transferred within the frontal zone of a well-developed low is derived for a 2- to 3-day event. This number multiplied by the average number of lows gives a coarse estimate of the contribution to the annual stratospheric ozone outflow in the northern hemisphere (1.5¿1036 molecules yr-1). It is smaller than the total outflow (3.5--6.1036 molecules yr-1) inferred from previous studies using radioactivity deposition or general circulation model, which includes the additional impact of cutoff lows. ¿ American Geophysical Union 1991