The degree of permeability of the alveolar-capillary barrier to low molecular weight solutes has been supposed to depend upon the radius of the epithelial pores. The bullous emphysema is a natural model in which this radius may increase because of the high tension in the bulla. Accordingly, the 99mTc-DTPA clearance may be expected to be faster than normal in lung fields including bullae. Results of the present study in which 99mTc-DTPA clearance was measured in six patients with bullous emphysema, seem to confirm this hypothesis at least to some extent. In fact, either increased or normal 99mTc-DTPA clearances were found over the lung fields including bullae communicating with the bronchial tree. Differences in both the size and the inner tension of the bullae as well as in the anatomical and functional conditions of the surrounding parenchyma are likely to account for such varying behaviour.