Mitral regurgitation (MR) related to chronic degenerative valvular disease is the most important cause of heart failure in dogs. Ultrasound examination of the heart can identify valve lesions, confirm the presence of valvular regurgitation, document cardiac remodeling, estimate intracardiac pressures, and quantify systolic ventricular function. These findings can influence prognosis or selection of medical therapy. Reductions in ventricular systolic function may be detected on serial echocardiographic examinations in some dogs with MR. However the changes in ventricular loading that accompany MR often complicate these measurements. For example, shortening and ejection fractions are often increased in severe MR, even in the setting of congestive heart failure. Echocardiography with Doppler is also used to assess ventricular diastolic function and filling pressures. This information helps predict the risk of congestive heart failure. However these findings are often rendered ambiguous by age-related impairment of ventricular relaxation, elevations in left atrial pressure due to MR, and effects of volume overload on myocardial tissue velocities. These factors limit the usefulness of ventricular filling and tissue velocities, as well as derived ratios such as the E/E' ratio, for predicting congestive heart failure in MR. More advanced Doppler and tissue echocardiographic methods, as well as prospective clinical studies, are needed to reduce the ambiguity involved with assessment of ventricular function and filling pressures in the setting of MR.