Beta adrenergic blocking drugs in hypertrophic cardiomyopathy provide symptomatic relief but their effect on long-term prognosis is uncertain. Thirty patients were studied non-invasively by simultaneous recordings of echocardiogram, apex-cardiogram, phonocardiogram, and electrocardiogram in order to assess diastolic abnormalities on and off oral beta adrenergic blocking drugs. While on treatment these patients had a mean dose of propranolol 200 mg/day. The treatment was stopped for one week and then non-invasive assessment was repeated. The following diastolic time intervals were studied: isovolumic relaxation period (A2-mitral valve opening); rapid relaxation period (A2-O point of the apexcardiogram), and the period from mitral valve opening to the O point of the apexcardiogram (Mo-O) when most of the filling of the left ventricle occurs. The prolongation of the rapid relaxation period reflects a reduced rate of fall of the left ventricular pressure when the pressure differential does not change between A2 and the O point of the apexcardiogram, and in this study this period was prolonged in 19, shortened in eight, and remained the same in three patients after beta blockade. The Mo-O point was prolonged in 22, shortened in seven, and was unchanged in one patient after beta adrenergic blocking drugs. All these results were independent of heart rate. In conclusion the response of diastolic time intervals to beta blocking drugs in hypertrophic cardiomyopathy was variable but there was a significant number of patients in whom the time available for filling of the left ventricle was prolonged, suggesting better filling possibly because of improved distensibility of the left ventricle after beta adrenergic blocking drugs.