AbstractRegional ventilation has been measured in 17 healthy volunteers, and 24 patients with chronic obstructive lung disease during tidal breathing using133Xe.The wash-in and wash-out of133Xe were recorded by a gamma camera interfaced to a small digital computer. Regional ventilation was calculated as the distribution of tidal volume per unit lung volume—a measure of relative ventilation—and from the wash-out curves as the fractional exchange of air per second.Determination of the regional fractional exchange of air showed a significant difference between the patients with chronic obstructive lung disease and normal subjects for all regions. The distribution of tidal volume per unit lung volume did not effect such a clear separation.Significant correlations were found between the wholelung fractional exchange of air in the patients with chronic obstructive lung disease and their FEV1r=0·70, MMFRr=0·70, FVCr=0·56 and FEV1/FVCr=0·57.It is suggested that measurement of regional ventilation as the fractional exchange of air is more realistic than methods that determine relative ventilation or only make use of the early part of the wash-out of133Xe.