Chest radiography and high resolution computed tomography (HRCT) were used to evaluate the degree of lung involvement in 38 adult cystic fibrosis patients with unusually mild pulmonary disease. Abnormal features were present on the chest radiograph and/or high resolution CT scans of 35 patients, while in three patients both investigations were normal. Thickening of the wall of proximal right upper lobe bronchi was the earliest abnormal feature on HRCT. The commonest abnormal feature in these patients, including those whose chest radiograph was normal, was mild, uniform dilatation of proximal bronchi. The lumen dilatation was always less marked than the degree of thickening. Ten of the patients were first diagnosed in adult life. Although HRCT scans were normal in two of these patients, there were features suggestive of early pulmonary disease in the remaining eight.