Regional lung function was assessed by radiographic methods, by regional function studies using xenon-133 scans, and by lobar sampling with a mass spectrometer flow-meter at bronchoscopy in 12 patients who subsequently had bullae resected at operation. The information given by these three methods of regional assessment was subsequently compared with the findings at operation. When only one lobe was abnormal on the radiographs, these alone were adequate to locate the major site of the emphysema and the regional tests gave relatively little extra information. The xenon scan was sometimes helpful in assessing the state of the remaining lung, but this information could be deduced from the radiographs and overall lung function tests, especially the carbon monoxide transfer and mechanical measurements. Bronchoscopic sampling was helpful in determining whether the affected lobe was acting as a ventilated dead-space. When more than one lobe was affected the regional function tests supplemented the radiographs in defining the site of bullous change as well as locating dead space. Xenon scans, although widely employed for such preoperative assessments, added little to the topographical information obtained by careful radiology. The combination of radiology, lobar sampling, and overall function tests is recommended for assessing which emphysematous patients are likely to benefit from surgery.