The usefulness of CT with 4-mm-thickness cuts at 5-mm intervals in the diagnosis of bronchiectasis was studied by comparing the results of CT with those of bronchography in 323 segmental bronchi in 20 patients. No bronchiectasis was found on either study in 222 segmental bronchi. Of the 101 segmental bronchi showing bronchiectasis on bronchography, CT correctly identified bronchiectasis in 98 segments (97%). Bronchography showed cystic bronchiectasis in 35 and varicose bronchiectasis in 14 segmental bronchi, all of which were correctly identified on CT. Of the 52 segmental bronchi showing cylindrical bronchiectasis on bronchography, CT correctly identified these changes in 49 segments but failed to detect it in three. The presence of interstitial lung disease with emphysema and very localized mild bronchiectasis in one case each made the identification of cylindrical bronchiectasis difficult and caused the false-negative results. When bronchography is used as the criterion, CT with medium-thickness cuts and medium slice intervals appears to be 100% specific for all types of bronchiectasis, 100% sensitive for cystic and varicose bronchiectasis, and 94% sensitive for cylindrical bronchiectasis.