Our purpose is to define the morphological patterns of interstitial lung disease on CT and to evaluate the diagnostic impact of high-resolution technique (HRCT). Sixty-six consecutive patients with proven interstitial lung disease were considered in our study. The basic morphological patterns include: a) large reticular pattern (10-25 mm); b) small reticular pattern (2-3 mm); c) intermediate reticular pattern (5-10 mm) with cystic dilatation of distal airspaces; d) nodular pattern; e) high-density parenchymal areas. Topography of the involved areas (peripheral, middle and axial compartments), and lesion distribution with reference to the secondary pulmonary lobule (centrilobular, perilobular, panlobular, bronchovasal) are additional diagnostic criteria. The identification and correlation of these three parameters proved very useful in limiting the range of diagnostic possibilities to interstitial disease, obviously considering clinical data. In our series a correct CT diagnosis was obtained in 57 of 66 cases (86.36%). In 24 of them (36.34%) diagnosis corrected a previous erroneous or generic clinical suspicion. Our data suggest that HRCT is indicated in interstitial lund disease when neither clinical nor radiographic findings allow a specific diagnosis to be made.