A semi-automatic method is described for tracking the contour of the diaphragm in lateral X-ray image sequences. It is part of a method being developed to quantify diaphragm function non-invasively. Initialization is achieved by interactive segmentation of the diaphragm contour and identification of landmarks. The method relies on modelling the contour using an augmented active contour model incorporating anatomical constraints. The contour is then tracked iteratively across successive frames using reduced-search space dynamic programming. The method has been tested on several image sequences and preliminary results are promising with an 85.3 % tracking accuracy for one such sequence.