Airway hyperresponsiveness (HR) is one of the clinical characteristics of bronchial asthma and COPD. HR of COPD has been known to correlate with progress of pulmonary function in COPD. In order to evaluate the necessity of HR measurement, HR of COPD alone, COPD with asthma, asthma and control subjects was compared. Patients with COPD alone exhibited weak HR, while COPD with asthma represented severe HR, which was the same degree with asthmatics. From these results, we should be cautious in measurement of HR in COPD. Especially patients with FEV1 less than 1 L should be prevented from HR test.