A series of 20 patients with pure and severe major aortic incompetence was studied, and three different groups were distinguished: group I, with no previous defect in function; group II, with cardiac failure as a result of aortic incompetence of more than 5 years' standing; group III, with cardiac failure secondary to acute aortic incompetence. The internal diameter of the left ventricle and the mode of closure of the mitral valve were studied by echocardiography, and compared with the clinical and haemodynamic findings. Left ventricular dilatation appeared early, and occurred to the same degree in the three groups. The diastolic diameter of the left ventricle showed a fairly close correlation (r equals 0.61) with the degree of regurgitation as measured from clour dilution curves; by contrast, the length of time the aortic incompetence had been present had little influence on the degree of dilatation of the left ventricle. Premature closure of the mitral valve was significantly associated with a raised LVEDP and a low systolic index; it represents a defect in the compliance of the left ventricle, and is a poor prognostic factor because of the clinical progression into very rapid and severe heart failure. Moreover, there is a distinct aetiological factor (Infective endocaditis) in the most marked forms of premature closure. The other ecocardiographic findings (appearances of the aorta, mitral echo, slope EF of the mitral valve) give no information on the degree of tolerance to the cardiac defect.