Cardiomegaly is a common but nonspecific finding in the emergency department. The etiology may be explained by left ventricular dilation, biventricular dilation, right ventricular dilation, or pericardial abnormalities, or it may be found to be spurious on the echocardiogram. Rarely, isolated abnormalities of the atrium, particularly the left atrium, may cause abnormalities on the chest radiograph but will not cause true cardiomegaly. We reported a 37-year-old woman who presented with persistent palpitation of a 6-hour duration. An electrocardiogram revealed atrial fibrillation with rapid ventricular response. A chest radiograph showed cardiomegaly with unusual prominence of the left heart border. A subsequent transthoracic echocardiography demonstrated an echo-free chamber adjacent to the left lateral wall of the left ventricle. The diagnosis of a rare huge left atrial aneurysm was finally established by multidetector-row computed tomography, which had been seldom used as a diagnostic tool in the past. Patients with this cardiac anomaly usually are asymptomatic until the second or third decade of life, when the aneurysms can reach remarkable sizes and are often complicated with atrial tachyarrhythmia or thromboembolic events. Early surgical intervention is advised even in asymptomatic patients to prevent the occurrence of myocardial dysfunction, atrial fibrillation, and systemic embolism.