The Flemish Ardennes (W Belgium) are known to be affected by deep-seated landslides. The assessment of the landslide reactivation hazard requires understanding the driving processes and delimiting precisely not only the landslide boundaries but especially that of their most active parts. Precise 3D models of 13 landslides were produced by digital stereophotogrammetry using aerial photographs of different dates. Dealing with photographs at the scale 1:25000 or larger, we obtained for each model an accuracy better than 0.5m. As a first result, the main size parameters of the landslides (width, length, depth, volume, .) are easily computed. Moreover, the obtained DTMs may be subtracted from each other in order to determine the apparent vertical displacement of each pixel during the interval of time considered. Provided that more than 2 epochs are documented, such DTMs not only supply precise information about distribution and style of the landslide activity but may also point to temporal variations in this activity. The subtraction of DTMs allows us to give an estimation of the volume of the 'uplifted' and 'collapsed' terrains between two epochs.