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Simultaneous recordings of reflected ultrasound from the anterior mitral leaflet and left ventricular outflow, the ECG, the phonocardiogram, and a recording of the carotid artery pulse were obtained in six patients with hypertrophic obstructive cardiomyopathy. Abnormal sharp systolic anterior movement (SAM) of the mitral leaflet was observed. This movement began with the onset of ventricular ejection and reached a peak with the initial peak in the arterial pulse. The leaflet was apposed to the interventricular septum up to 60% of the ejection period. In the latter part of systole as the mitral leaflet moved away from the interventricular septum, the arterial pulse showed a second systolic wave. Onset of SAM coincided with onset of the systolic murmur. Spontaneous variations in amplitude of SAM coincided with alterations in contour of the arterial pulse and in the intensity of the murmur. Administration of methoxamine to four patients resulted in disappearance of SAM. In one patient following surgery, the SAM of the mitral leaflet was noted only in the post-ectopic beats. This specific abnormality of mitral leaflet movement represents the localization of dynamic outflow obstruction in hypertrophic obstructive cardiomyopathy.