In previous articles, I have emphasized that interstitial lung disease/pulmonary fibrosis first requires a clear diagnosis. First, is it interstitial lung disease or pulmonary fibrosis or not? Second, if so, what are the possible types and etiologies? There are many types of interstitial lung disease/pulmonary fibrosis, and it is not easy to make a reasonable diagnosis. Therefore, clinically, we sometimes encounter misdiagnosed cases that are not interstitial lung disease/pulmonary fibrosis and are mistakenly thought to be. And more commonly, after the diagnosis of interstitial lung disease/pulmonary fibrosis is made, the type and possible causes are not further explored, and only a generalized clinical judgment can be made, and thus no reasonable choice of therapeutic drugs can be made, and the prognosis is not as reasonable as it could be. Recently we diagnosed a new case of interstitial lung disease, which illustrates how important it is to establish a diagnosis. A male patient in his forties was found to have diffuse shadows in both lungs during a local chest radiograph and was treated ineffectively with antimicrobial agents. After careful analysis of the case data, we concluded that the diagnosis of interstitial lung disease was clear, but did not match with any of the several types of interstitial lung disease/pulmonary fibrosis commonly seen today. We performed electronic bronchoscopy and transbronchial lung biopsy after completing various tests regarding interstitial lung disease, and the results were still inconclusive. Therefore, after consultation with the patient, a thoracoscopic lung biopsy was performed and the diagnosis of “immunoglobulin G4-associated interstitial lung disease” was confirmed. This disease has not been officially reported in China. After diagnosis, the patient was treated accordingly, and follow-up revealed significant resorption of the lung shadow (see above). There is another disease that is sometimes misdiagnosed as idiopathic pulmonary fibrosis. For example, an elderly male patient with cough and dyspnea was examined in an outside hospital and a chest CT was taken, showing honeycomb-like changes in both lungs. After hospitalization, we found that the patient had been having low fever and abnormal urine examination, so we considered the possibility of other diseases. Finally, after laboratory tests, the diagnosis of pulmonary small vessel vasculitis was confirmed, and the interstitial lung changes were significantly absorbed and improved after treatment (see the figure below). Figure 1 Before treatment: cool lung CT changes resembling pulmonary fibrosis Figure 2 After treatment: significant absorption of interstitial lung lesions