Potentially lethal arrhythmias (PLA) are common in hypertrophic cardiomyopathies and are implicated in sudden death. We have demonstrated that propranolol is effective in controlling symptoms, but have not previously evaluated its effect on PLA. Our protocol for long-term management has been applied to 50 patients followed for 2 to 14 years (mean 5.9 years). No patient was excluded because of disease severity or complications. The only reason for exclusion was non-compliance with the protocol. At entry, Holter monitoring revealed 16 PLA in 13 patients (sustained supraventricular tachycardia (SSVT) in two, sinus node dysfunction (SSS) in three, paired beats (PB) in five, and ventricular tachycardia (VT) in six). All patients were initially begun on propranolol, unless a different beta blocker was indicated. Dosage was adjusted to a standing heart rate of 60 beats/min unless adverse drug effects developed. Holter monitoring during follow-up identified 24 new PLA in 21 patients (SSVT = 1, SSS = 4, His-Purkinje disease = 3, VT = 10, and PB = 6). The total number of identified PLA is now 40 in 30 patients. Concurrent symptoms of syncope, presyncope and palpitations were of limited value in identifying PLA because of a lack of predictive accuracy (76%, 64% and 29%, respectively). Invasive electrophysiologic study or dosage adjustment proved that beta blockade was not responsible for the conduction system disorders observed in 10 patients requiring pacemaker insertion. beta-Blockade completely suppressed VT in two patients, PB in five, and SSVT in two.(ABSTRACT TRUNCATED AT 250 WORDS)