Twenty-five patients with hypertrophic obstructive cardiomyopathy confirmed by clinical, echocardiographic and haemodynamic investigations were treated with high dose propranolol (320 to 800 mg/day, average 420 mg/day) and assessed after an average follow-up period of 25 months. The effects of treatment were assessed by interrogation in all patients and by exercise testing before and after propranolol in 19 cases. Comparative echocardiographic (21 patients) Holter (20 patients) and catheter studies (14 patients) were also performed. The cardiovascular mortality rate during the study period was nil. All patients were symptomatic before treatment; 9 became asymptomatic and 13 patients were improved, the average functional score decreasing from 5.16 +/- 2.15 to 2.28 +/- 1.49 (p less than 0.001). The persistence of severe symptoms led to withdrawal of the beta-blocker in 2 cases. There was a parallel improvement in maximal work capacity during exercise stress testing (96 +/- 27 watts vs 117 +/- 30 watts, p less than 0.01). A resting intraventricular pressure gradient was recorded in 12 of the 14 patients undergoing repeat catheter study which decreased after propranolol from an average of 66.75 +/- 32.72 mmHg to 42.75 +/- 37.6 mmHg (p less than 0.05). Left ventricular end diastolic pressures remained unchanged. The change in pressure gradient did not correlate with the symptomatology and there were no associated echocardiographic changes. The Holter monitoring did not show any improvement of ventricular hyperexcitability under propranolol: the number of patients with complex ventricular arrhythmias remained unchanged (7 patients).(ABSTRACT TRUNCATED AT 250 WORDS)