Mild enlargement of the left atrium is defined as an internal diameter of the atrium exceeding 35 mm. Mild enlargement of the left atrium can occur in many normal people, and some are idiopathic enlargement of the heart structure, in which case observation is sufficient. However, many cases are caused by pathological factors, such as long-term pulmonary heart disease, dilated cardiomyopathy, coronary artery disease, and other conditions, which can result in mild enlargement of the left atrium. After mild enlargement of the left atrium occurs, the main option is to treat the cause. In the case of coronary artery disease, the main focus is to improve ischemia, and in the case of pulmonary artery disease, the main focus is to improve cardiopulmonary function. Mild enlargement of the left atrium can easily cause arrhythmias, especially atrial arrhythmias such as atrial flutter and atrial fibrillation, and patients must pay attention to them. If atrial premature beats or atrial flutter or atrial fibrillation occur, active symptomatic treatment is required. Many left atrial enlargements are not easily reversed after they occur, and regular checkups are needed.