The chronic inflammation of bronchial asthma is characterized by swelling of the subepithelial mucosa. However, it is difficult to assess subepithelial edema clinically. We report the case of a patient with asthma whose subepithelial edema was evaluated by endobronchial ultrasonography. Receiving montelukast 10 mg/day for 2 weeks, a 42-year-old man with mild, persistent asthma had his symptoms controlled by beta2-inhalation alone. Pretreatment endobronchial ultrasonography revealed subepithelial thickening in the right main stem bronchus, with a low absorption area suggestive of edema. Two weeks of montelukast therapy diminished the amount of subepithelial edema. Endobronchial ultrasonography is a promising technique for determining subepithelial edema in the asthmatic airway.