Diastolic mitral regurgitation is an unusual phenomenon, which occurs when the left ventricular pressure exceeds the left atrial pressure during diastole. Four patients with angiographically documented diastolic mitral regurgitation were studied by synchronized pressure and cineangiographic techniques, and later underwent successful aortic valve replacement. Two had severe aortic regurgitation without associated lesions. The third had mild aortic regurgitation associated with severe aortic stenosis, and the fourth had moderate aortic regurgitation and severe myocardial dysfunction. Aortic regurgitation thus "overfills" the left ventricle, either by a large increment in left ventricular filling or by lesser degrees of increment in a noncompliant (hypertrophic or cardiomyopathic) ventricle. It is important to recognize diastolic mitral regurgitation, since it implies a severe hemodynamic state and can be relieved without mitral valve replacement.