Chronic therapy with propranolol has been shown to reduce the incidence of sudden death in patients with hypertrophic cardiomyopathy (HCM). However, the long-term effect of β blockade on exercise capacity has not been studied adequately. Therefore, 32 patients with HCM (21 men), mean age of 47 years (range = 14 to 80 years), were evaluated for dyspnea and chest pain and underwent stress testing (ST) prior to therapy. At entry, ST was contraindicated in four patients, because of heart failure (three patients) and sustained supraventricular tachycardia (one patient). The remaining patients completed 4·9±3· 2 min (mean±S.D.) of the Bruce protocol with a functional aerobic capacity (FAC) of 51±28%. All were placed on propranolol, unless a β blocker with other characteristics was indicated. Dosage was adjusted to achieve a standing heart rate of 60 beats/min unless adverse effects occurred. At last follow-up, 25 patients were receiving 501±147 mg propranolol/day while the remainder received nadolol or metoprolol. On the most recent ST, patients exercised 6·6±3· 1 min (38% increase), while mean FAC increased by 24% (both P<005). The FAC improved by more than 15% in 21, by less than 15% in five, was unchanged in five and was worse in only one, a noncompliant patient. The 21 patients with an FAC increment <15% exercised longer at entry than the remaining 11 (6·6±3·3 vs 3·9±2·8 minutes, P<005). The mean peak ST blood pressure-heart rate product of the group decreased from 26 550 to 17 898 (P<005), while the symptom scores of dyspnea and chest pain declined from 2·2±0·8 to 0·8±0·7 and from 1·4±1·0 to 0·3±0·8, respectively (both P<0001). We conclude that β blockade produces sustained improvement in exercise capacity. Improvement was greatest in those with the least initial impairment, and appears to be related to a reduction in blood pressure—heart rate product.