Diagnosis in non-immunosuppressed subjects with chronic diffuse infiltrative lung disease is a progressive process. Because of the large number of diseases involved, the relative frequency of certain causes (for example sarcoidosis, primary pulmonary fibrosis, carcinomatous lymphangitis) is of prime importance. Generally, history taking and physical examination are essential to guide exploratory examinations required for diagnosis. Standard radiography often gives the first indication of chronic diffuse infiltrative lung disease, but the recent development of high-resolution computed tomography which provides a better visualization of the predominant elementary aspects and their anatomic distribution has been a major step forward. Bronchoalveolar lavage is also highly contributive and although certain diagnosis may not be achieved, the type of cells involved in the development of alveolitis can be identified. Pathology examination of a surgical biopsy specimen is sometimes required for certain diagnosis. Videothoracoscopy is generally used and should be guided by CT scan data to optimize the probability of sampling in areas of "active" inflammation. Indications for videothoracoscopy must be made on an individual basis according to the team's experience with this type of pathology.