Standard radiography of the chest shows pulmonary infiltration and is determinant for the diagnosis of chronic diffuse infiltrative lung diseases. Since lung biopsy is an invasive exploratory technique, using computed tomography (CT) in the diagnostic approach is perfectly justified as a direct prolongation of plain radiography. CT identifies patients who, in spite of normal chest X-ray films, do have pulmonary infiltration. It is superior to radiography in suggesting the correct aetiological diagnosis, as it provides signs that are more discriminant than the radiographic signs. It also gives a better approach to the localization of lesions, thereby helping in guiding lung biopsies. Finally, once the diagnosis has been made CT remains useful in evaluating the effects of treatment of pulmonary lesions and in detecting possible complications.