Chronic mitral valvular insufficiency (CMVI) is the most common heart disease in dogs. The prevalence of CMVI is age-dependent. CMVI is usually affected to small to medium size breeds. It is more prevalent in males than females. The characteristic lesions of CMVI are caused by an acquired chronic structural degeneration of the mitral valve defined as endocardiosis or myxomatous degeneration. The main clinical signs are cough, respiratory distress, weakness and pleural effusion and ascites by secondary right-sided heart failure. The most prominent clinical finding is a systolic heart murmur. The thoracic radiography and echocardiography are useful methods in diagnosis of CMVI. Thoracic radiographic findings are left atrial enlargement, left main stem bronchial compression and pulmonary edema. Echocardiography is confirmed to increased left atrial and ventricular dimension, mitral regurgitation, mitral valve thickening and abnormal movement. Thoracic radiography and echocardiography are used to obtain a definite diagnosis of CMVI, and then to study the progression of the condition.