Results of computed tomography of the lung performed at two levels in upper lung zones at full inspiration and full expiration were compared with results of tests of ventilatory function, lung mechanics, and single breath carbon monoxide diffusing capacity in 64 subjects, many of whom had some form of airflow obstruction. From the CT scans, the mean percentage of pixels in the range -900 to -1,024 Hounsfield units, or pixel index, was determined for each subject. The highest correlations of pixel index with physiologic variables consistent with a diagnosis of emphysema were observed for CT taken at full expiration. In some subjects, the inspiratory CT would give a "false positive" for emphysema when the hyperaeration observed at inspiration was not observed at expiration. We believe that the CT scan taken at full expiration can effectively reveal the abnormal permanent enlargement of airspaces which defines emphysema and provides a noninvasive method of assessing lung morphology in the living human subject.