Data of more than 30 years of ozone soundings at Uccle (5048'N, 421'E) are used to investigate the features of the vertical distribution of ozone. First results show that there is a general increase in tropospheric ozone over the period. However, the largest increases in the free troposphere occurred at the beginning of the timeseries (i.e. during the 1970's), while on the other hand, the largest increases in the boundary layer are seen later (during the 1980's). To get a better picture of the reasons for the trends, studies relating trends to meteorological conditions were started. An attempt was made to select different air masses. As a first proxy for the air mass, soundings were divided in different classes according to the wind direction. It turns out that the most important trends are found with winds from the East and the Northwest. Also an effort is made to calculate trends in function of a climatological season. Therefore, trends have been calculated with respect to the ground and with respect to the tropopause, to exclude the variability, due to changes of the altitude in tropopause height.