Aim: The use of β-blocker therapy in asymptomatic patients with hypertrophic cardiomyopathy is controversial. This study evaluates the effect of lifestyle changes and high-dose β-blocker therapy on their exercise capacity. Methods and results: A total of 29 consecutive newly diagnosed asymptomatic patients with familial hypertrophic cardiomyopathy, median age 15 years (range 7–25), were recruited. In all, 16 patients with risk factors for sudden death were treated with propranolol if no contraindications, or equivalent doses of metoprolol; 13 with no risk factors were randomised to metoprolol or no active treatment. Thus, there were three treatment groups, non-selective β-blockade (n=10, propranolol 4.0–11.6 mg/kg/day), selective β-blockade (n=9, metoprolol 2.7–5.9 mg/kg/day), and randomised controls (n=10). All were given recommendations for lifestyle modifications, and reduced energetic exercise significantly (p=0.002). Before study entry, and after 1 year, all underwent bicycle exercise tests with a ramp protocol. There were no differences in exercise capacity between the groups at entry, or follow-up, when median exercise capacity in the groups were virtually identical (2.4, 2.3, and 2.3 watt/kg and 55, 55, and 55 watt/(height in metre)2 in control, selective, and non-selective groups, respectively. Maximum heart rate decreased in the selective (−29%, p=0.04) and non-selective (−24%, p=0.002) groups. No patient developed a pathological blood-pressure response to exercise because of β-blocker therapy. Boys were more frequently risk-factor positive than girls (75% versus 33%, p=0.048) and had higher physical activity scores than girls at study-entry (p=0.011). Conclusions: Neither selective nor non-selective β-blockade causes significant reductions in exercise capacity in patients with hypertrophic cardiomyopathy above that induced by lifestyle changes.