The external carotid pulse and the phonocardiogram were recorded in a 48-year-old man with idiopathic hypertrophic subaortic stenosis and atrial fibrillation. The degree of obstruction of left ventricular outflow was assessed by the depth of the mid-systolic dip, the length of the left ventricular ejection time and the intensity of the systolic murmur. The correlation found between the degree of outflow obstruction and the length of the preceeding diastole was interpreted in terms of the Frank-Starling mechanism, i.e. augmented diastolic filling led to an increase in the force of contraction and hence to an increase in muscular obstruction to outflow. The observation that this relationship held also for long diastoles suggested that the left ventricle was not completely filled during diastoles of middle length, probably because of the low diastolic distensibility of the stuff and hypertrophic myocardium.