To evaluate the prognostic role of left atrial (LA) volume in hypertrophic cardiomyopathy (HCM), LA volume was measured at baseline and during follow-up in 140 patients with HCM. Unfavorable outcome, defined as occurrence of sudden death, heart transplantation, or invasive reduction of obstruction, developed in 16 patients. In patients with enlarged LA volume (>27 mL/m(2)), there was an increased risk for unfavorable outcome (P = .0152). Patients with normal LA volume at baseline in whom volume increased more than 3 mL per year (fast dilating LA volume) had a worse prognosis than patients with normal and stable volume (P < .001) and similar to patients with dilated LA volume at baseline (P = not significant). LA volume dilated at baseline, fast dilating LA volume, and New York Heart Association functional class were independent predictors of unfavorable outcome development (odds ratio: 11.453; P = .021, P = 2.019, P = .020, respectively). The assessment of LA volume at baseline and during follow-up adds information regarding prognosis in patients with HCM.