In order to characterize the appearances of bronchiolitis obliterans on high-resolution CT (HRCT) and to relate the extent of HRCT abnormalities to pulmonary function tests, the HRCT scans of 18 patients with bronchiolitis obliterans were examined by two independent observers. The underlying causes for the development of bronchiolitis obliterans were penicillamine therapy (n = 5), previous infection (n = 4), graft-vs-host disease (n = 3), and miscellaneous causes (n = 6). Abnormal HRCT findings were present in all cases. The commonest HRCT abnormalities consisted of patchy areas of decreased parenchymal attenuation (n = 15), subsegmental (n = 12) and segmental (n = 6) bronchial dilatation and centrilobular branching structures (n = 5). There was no significant correlation between the extent of abnormalities and the static lung volumes or the impairment in gas transfer (all P values > 0.05). The only significant correlation was between the number of segments with subsegmental bronchial dilatation and the forced expiratory volume in one second (Wilcoxon r = 0.61, P < 0.01). We conclude that the great majority of patients with bronchiolitis obliterans have HRCT abnormalities. However, there is poor correlation between the extent of abnormalities on HRCT and functional impairment.