Almost 40% of patients with heart failure (HF) have preserved left ventricular (LV) ejection fraction (EF) and prognosis similar to those with reduced EF. Data on prognostic markers in such patients are limited. We analyzed the prevalence and prognostic value of left atrial (LA) size in this condition. 89 normal subjects (Group I), 38 asymptomatic hypertensive patients (Group II) and 183 HF patients with preserved EF (EF >45%) (Group III) were studied. LA diameter (LAD), LV diastolic (LVD) and systolic (LVS) dimensions and mass (LVmass) and EF were measured. E and A wave velocities and E/A were measured. The primary end point was all cause mortality in group III patients. Groups did not differ in age, gender or EF. Group III patients had larger LAD (4.6+-1.0 cm) compared with both Group I (3.7+/-0.6) and Group II (3.7+/-0.5 cm) (p<0.0001). A markedly enlarged (arbitrarily defined as LAD higher or equal 5 cm) had an odds ratio of 34 (95% CI 8-144) in distinguishing HF patients from normals. After a mean follow-up period of 29+/-27 months, 40 patients (21.9%) died. In Cox univariate analysis, NYHA class (HR 2.8 95% C.I. 1.8-4.3; p<0.0001), diastolic blood pressure (DBP) (HR 0.92 95% C.I. 0.88-0.96; p<0.0001), age (HR 1.059 95% C.I. 1.01-1.11; p=0.02) and LAD (HR 1.72 95% C.I. 1.27-2.3; p=0.0005) were predictors of mortality. LAD predicted survival independently of other variables. The left atrium is frequently dilated in HF patients compared with controls despite similar EF. LAD showed powerful prognostic value independent of clinical variables.