Left ventricular filling was studied by echocardiography and cineangiography in 37 patients with hypertrophic cardiomyopathy with obstruction before and during long-term oral beta blockade (166 +/- 61 mg propranolol for 15.6 +/- 21 months on average) in order to determine the mechanism of symptomatic improvement in these patients. This study confirmed that clinical improvement is related to a great degree to the improvement in the parameters of left ventricular filling. In addition, two groups of patients may be identified: the "reactors", that is to say the patients who remained or became asymptomatic, characterised by a significant improvement in the indices of left ventricular filling (maximum filling velocity p < 0.01, duration of rapid filling p < 0.05, left ventricular compliance--volumic kGaasch p < 0.001, parietal ks Mirsky p < 0.01). The "non-reactors" were characterised by very poor indices of left ventricular filling. These abnormalities are principally related to severe and irreversible changes in left ventricular and mitral valve morphology.