Computed Tomography scanners can be calibrated to provide reproducible measurements for the quantitative analysis of lung density. We measured in square centimeters the pulmonary density areas for each interval of values expressed in Hounsfield Units and obtained some histograms representing the "respiratory curves" for full inspiration and full expiration. The study population had been divided into three groups according to their clinical conditions. Group A: 12 normal subjects, aged 20-40, non-smokers; Group B: 23 patients, aged 40-65, smokers, asymptomatic or slightly asthmatic; Group C: 19 patients, aged 55-75 hospitalized with chronic obstructive pulmonary disease. The respiratory curves obtained at full expiration appeared to be completely different in the three groups resulting more significant than those obtained at full inspiration. We conclude that the histograms obtained by measuring the pulmonary density areas in full expiration at preselected intervals of HU values can be useful as a screening test as well as to quantify pulmonary emphysema.