High resolution computed tomography (TDM-HR) is now the technique of choice in the diagnosis and management of diffuse infiltrative lung disease (PID). After a brief review of the technique the authors describe the normal appearance; anatomical observations and the in vivo findings have shown that TDM-HR allow for the exploration of details of structure down to the second pulmonary lobule. Thus, through the alterations that are transmitted in the lobular area, and from its contents and its limits, PID has led to the elaboration of a new semeiology. The authors review the basic computed tomographic images and correlate these in each case with the histological evidence. The spatial distribution and the time sequence of the elementary images are the two other terms in the diagnostic equation of PID. The spatial distribution of several elementary images presents in TDM-HR a superior aetiological pointer to that which is furnished by thoracic radiographs; and the time sequence may furnish a useful indication as to the progress of the treated disease. Sarcoidosis, histiocytosis X, idiopathic interstitial fibrosis and lymphangitis carcinomatosis would serve as examples. Nevertheless, the authors point out that it would be dangerous during the period of evaluation to prematurely extend to all cases of PID conclusions which are only possible to make at present in a restricted number of disorders.