We have calculated the effective time (t eff) of the forced expiratory spirogram in healthy non-smokers, in patients with simple chronic bronchitis, and in patients with chronic obstructive lung disease. Effective time was obtained by dividing the area underneath the forced expiratory spirogram by the forced vital capacity (FVC) and is equal to the time constant of lung emptying if the FVC-time curve is a single exponential. In 75 healthy non-smoking adults, t eff showed a linear increase with age but no difference between the sexes. In 37 patients with low forced expiratory volume in one second (FEV1)/FVC ratios, t eff, as expected, was increased. In 19 subjects with simple chronic bronchitis and normal FEV1/FVC ratios, t eff was longer than or at the upper limit of 2 SE above the mean for healthy subjects of the same age. A comparison of t eff with FEV1/FVC suggested that in most normal subjects and patients the forced expiratory spirogram was a multiple exponential and that t eff was a more sensitive index for the evaluation of airway obstruction than the FEV1/FVC ratio.