PURPOSE: To correlate radiologic findings with clinical findings in patients with a mosaic patten of lungattenuation, as seen on thin-section CT. MATERIALS AND METHODS: Thirty-one cases in which a mosaic pattern oflung attenuation was detected on combined expiratory and inspiratory scans of thin-section CT were retrospectivelyanalyzed. Cases involving infiltrative lung disease were excluded. Both thin-section CT and clinical findings wereanalyzed and the relationship between the extent of the area of hyperlucency -as seen on expiratory scan- andphysiologic parameters was evaluated. The subjects were 10 men and 21 women ranged in age from 25 to 76 (mean 50)years. RESULT: Twenty-nine patients with small airway disease, [chronic bronchitis and/or bronchiolitis(n=11),bronchiectasis(n=8), bronchial asthma(n=8), mycoplasmic pneumonitis(n=1) and hypersensitive pneumonitis(n=1),] andtwo patients with pulmonary vascular disease, [chronic pulmonary thromboembolism(n=1) and stenosis of the leftupper pulmonary artery(n=1)] were included in our study. Commonly associated thin-section CT findings in the casesinvolving small airway disease(n=29) were bronchial wall thickening(n=25), nodular opacity(n=25), bronchial andbronchiolar dilatation(n=20) and small branching opacity(n=16). These findings were not observed in two patientswith pulmonary vascular disease, though bronchial wall thickening was seen in the patient with chronic pulmonarythromboembolism. At expiratory scan level, there was statistical correlation between FEV1/FVC and the number ofpulmonary segments(r= 0.982, p<0.05), but no correlation between FEV1/FVC and the percentage area ofhyperlucency(r=0.803, p>0.05). CONCLUSION: The mosaic pattern of lung attenuation seen on thin-section CT isindicative of various diseases, involving small airways such as bronchiolitis, bronchitis, bronchiectasis andbronchial asthma, and vascular lung disease. Bronchial wall thickening and nodular opacity can be associated withsmall airway diseases.