The contribution of bullous emphysema (BE) to the functional impairment of patients with concomitant diffuse emphysema (DE) and the confounding effects of BE on functional measurements were investigated. Twenty-nine patients (Group I), with BE and DE were compared with a group of patients without BE matched, among other criteria, for radiographic extent of DE (Group II). Group I showed significantly lower  , FEV  and DLCO values and higher MRC score than Group II. In Group I the radiographic extent of BE and the extent of DE did not predict the functional impairment. The FEV /FVC ratio in the subgroup with BE extent   of total lung volume was higher than in subgroups with BE extent   and 15%, respectively. In the same subgroups the correlation between DE and DLCO increased with the extent of BE. We conclude that BE contributes to the functional impairment of patients with concomitant DE. The confounding functional effect of bullae depends on BE extent: relatively milder obstruction can be observed with severe BE, whereas moderate BE causes modest deterioration of diffusing capacity, explaining the lack of functional-radiologic correlations in Group I. Therefore the computed tomographic scan is very useful in the work-up of BE with DE associated.