With 31%, mitral insufficiency (MI) represents the second most common valve lesion and is most commonly of degenerative etiology. Sudden death is rare, in asymptomatic patients with flail leaflet and in sinus rhythm it occurs at a rate of 0.8%/year, and increases to a rate of 4.8%/ year when atrial fibrillation occurs. The effective regurgitant orifice area (ERO) is an important determinator of severity of MI and is of prognostic importance. An ERO of >or=40 mm(2) implies severe MI. These patients require further evaluation with exercise testing to determine functional and objective symptom status and exercise hemodynamics to identify pulmonary hypertension at rest or during exercise. This is of particular importance in patients with absent tricuspid insufficiency. Coronary angiography is indicated when symptoms or pathologic hemodynamics occur during exercise. Surgery in asymptomatic, severe MI is recommended when ejection fraction drops to <60%; left ventricular end-systolic diameter (LVESD) increases to >45 mm (LVESD index >26 mm/m(2)), when atrial fibrillation occurs or pulmonary hypertension with PAP syst >50 mmHg at rest and >60 mmHg during exercise), and if the valve is reparable.Patients without an indication for surgery need careful cardiac follow-up. Patients who underwent surgery according to guidelines had an 8-year survival rate of 89%. Clinical assessment together with echocardiography and exercise hemodynamics determine the optimal timing of surgery in asymptomatic patients with severe MI.