Some studies have reported that area under the flow volume curve (AUFVC) can be an index of pulmonary function. However, the significance of AUFVC remains to be clarified. We have clarified that AUFVC reflects the momentum of expired air. Size of flow volume curve (= AUFVC) has been commonly recognized to be changeable visually in patients with asthma. To clarify whether size of flow volume curve (= AUFVC) is a useful index of the course of bronchial asthma, we compared the rate of improvement forced expiratory volume in 1 second (FEV1.0), peak expiratory flow rate (PEFR) or forced vital capacity (FVC) to the rate of improvement in AUFVC after admission in 20 patients with bronchial asthma. The rate of improvement in AUFVC positively correlated with the rate of improvement in FEV1.0, PEFR or FVC. AUFVC demonstrated more marked improvement than other indices. Since AUFVC reflects the momentum of expired air, bronchial asthma's patients could understand visually that the momentum of expired air decreased due to airway stenosis by presenting flow volume curve. Therefore, visual size of flow volume curve helped patients with asthma to understand the condition of asthma. Size of flow volume curve (AUFVC) may be useful index of the course of bronchial asthma.