Tracking of moving objects in an image sequence is an important task in many application (e.g. medical imaging, robotics). However, this task is usually difficult due to inherent problems that could happen in sequences (i.e. possible occlusion of the object, large interframe motion). In this paper, we describe a new approach to integrate a priori motion information into a level-set-based active contour approach. Specifically, we introduce a new constraint that enforces the conservation of the levels of the implicit function along the image sequence. This constraint is formulated as a motion prior energy and is used in a tracking algorithm. The method is validated quantitatively on a clinical application based on 10 echocar-diographic and 5 cine-MRI sequences (≈ 700 images).