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Transit time analysis of the forced expiratory vital capacity maneuver was applied to 37 patients with cystic fibrosis 8 to 22 years of age. This analysis divides the vital capacity into segments of air and assigns a transit time to each segment. The characteristics of the distribution of these transit times are used as measurements of pulmonary function. The quantities were compared with the forced vital capacity, 1-sec forced expiratory volume, ratio of 1-sec forced expiratory volume to vital capacity, peak flow, forced expiratory flow during the middle half of the forced vital capacity, maximal expiratory flow at 25 per cent of the vital capacity, specific airway conductance, and arterial PO2 of these patients. The standard deviation of the transit times was the most frequently abnormal. The mean transit time had the largest range of values in terms of its own standard deviation, and it was the best single estimate of over-all lung function because it correlated almost equally with large and small airway function. The coefficient of cariance of transit times was specific in detecting abnormality of small airways and was as sensitive in the detection of minimal lung disease as the standard deviation of transit times.

A simple expansion of the information in the maximal expiratory flow-volume curve was used to measure the convexity to the volume axis of maximal expiratory flow-volume curves produced by 11 asymptomatic smokers and 11 asymptomatic nonsmokers. The ratio of remaining volume to maximal flow was equal to the product of upstream resistance and chord compliance. This "instantaneous time constant" increased more near residual volume in the smokers. Using this method, plethysmographically obtained maximal expiratory flow-volume curves were more sensitive than spirometrically obtained maximal expiratory flow-volume curves. This method compared favorably with moment analysis and helium response of maximal expiratory flow as a separator of asymptomatic smokers and nonsmokers.