We describe a deformable model based technique for Cardiac MRI segmentation. The technique assumes that the data is available in the form of 2-D slices of the heart. An initial approximation of the boundary of the object of interest, say, the left ventricle, is specified in one of the slices, via a user-interface. The initial contour deforms to a contour with minimum energy, which is defined to be the correct ventricular boundary. This contour is then propagated to other slices, both in space and in time, to get the segmented volume at various instants in the cardiac cycle. This work is a part of our ongoing effort on cardiac MR analysis. The segmentation algorithm discussed here is intended to be a preprocessing stage for our work on volume computation and cardiac wall motion analysis. We have tested the segmentation algorithm extensively on over 500 images, and our clinical collaborators have found the results to acceptable, both qualitatively, and quantitatively. Our system is being installed for use in routine clinical practice.