['Left atrial (LA) enlargement is related to diastolic dysfunction and mitral regurgitation (MR), both of which are common in patients with hypertrophic cardiomyopathy (HCM). This study investigates the association between LA size and exercise capacity in patients with HCM.', 'All HCM patients who underwent a treadmill test with direct measurement of oxygen consumption (VO2) and a standard transthoracic echocardiography within 30 days in the years 2001-2003 were identified. Patients with significant comorbidities were excluded. Exercise capacity was defined as percentage of predicted peak VO2. Clinical and echocardiographic parameters were compared with those of a group of normal subjects.', "Compared with normal subjects, HCM patients had increased left atrial (LA) volume index (36 vs 21 mL/m2; P < .0001) and mitral E/e' ratio (14 vs 9; P < .0001); 27% of the patients had at least moderate MR. LA volume index demonstrated borderline correlation with exercise capacity (r = -.20; P = .06) but was an independent predictor of exercise capacity in a multivariate linear analysis, together with body mass index, heart rate at rest, and left ventricular end-systolic diameter. Including the parameters E/e' ratio or moderate or severe MR did not add incremental value to the model."] Is left atrial index a predictor of exercise capacity in patients with hypertrophic cardiomyopathy?