Many people have had chest films, lung CT, various blood tests and even fiberoptic bronchoscopy, but no problems can be found. Why am I so concerned about pulmonary function tests? What is the significance of pulmonary function tests in our clinical work? A 40-year-old middle-aged woman who had progressive wheezing for 4 years was initially treated as asthma, which had some effect at first, but later the symptoms became more and more severe and the effect of medication became worse and worse. We considered that the patient had a fixed obstruction in the upper airway and gave a 3D CT reconstruction to find a fixed narrowing in the upper airway. The second example is a middle-aged female patient with cough for 9 months and wheezing for 3 months, who was diagnosed with asthma and treated accordingly in a foreign hospital because of croup in the lungs, but the effect was not good and pneumonia appeared. After careful study of the lung CT, a high-density shadow was found in the left main bronchus. The patient later recalled having an experience of aspiration by mistake. The third example was an elderly male in his seventies who had been on a ventilator for a year and had a history of chronic obstructive pulmonary disease (COPD), but had no previous cough or sputum and no history of smoking. His biggest characteristic was that he could not leave the ventilator for one minute, and he had the feeling of not being able to inhale as soon as he left it, and his arterial blood carbon dioxide partial pressure could double from the normal value within one day. We sought the patient’s cooperation and performed a pulmonary function test at the bedside and found that the patient had severe restrictive ventilation dysfunction, which was completely different from the pulmonary function performance of COPD mentioned earlier, so we overturned the original diagnosis and considered respiratory muscle dysfunction, and further examination revealed that the patient had peripheral nerve impulse release dysfunction. These examples, although exceptional, reflect the special value of pulmonary function tests in diagnosis and differential diagnosis, so what kind of people need pulmonary function tests? Generally speaking, there are several aspects: (1) pulmonary function tests are a basic test whenever patients with chronic cough and unexplained dyspnea are involved; (2) assessment of the condition and evaluation of treatment efficacy in chronic airway diseases such as asthma and COPD; and (3) preoperative evaluation of patients undergoing major surgical procedures. So what is the significance of pulmonary function tests? (1) Firstly, it is to clarify the diagnosis. Many patients with chest tightness and shortness of breath may first consult the cardiology department, some consider cardiac insufficiency, but after active anti-heart failure treatment there is no improvement, and come for pulmonary function tests, which can then reveal that the patient has changes in airflow obstruction, hence the diagnosis of asthma or COPD, which improves significantly with the addition of corresponding drugs. (2) Secondly, the differential diagnosis, of which the above three cases are examples, can help us to effectively locate the corresponding examination from the face by examination, and also help us to find the real cause of the differential diagnosis. (3) Assessment of disease and efficacy. It is advisable for asthma and COPD patients to have a pulmonary function test every three months to understand the effect of treatment.