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The factors contributing to airflow obstruction in chronic respiratory diseases are frequently investigated by measuring an index of airways patency : 1) at various lung volumes, which permits to detect and quantify the obstruction, 2) at different static recoil pressures (Pst) to recognize abnormalities not related to a loss of elastic recoil. A diagram has been developed which facilitates the analysis and permits a more quantitative evaluation of intrinsic and extrinsic factors : the index of airways patency (I) is plotted simultaneously against Pst and against lung volume (expressed in % of predicted TLC), the two abscissae being scaled in such a way that the normal ranges are coincident for the two curves. Then, the distance from the I(Pst) curve to the normal range may be used to estimate the abnormalities due to intrinsic factors, and the distance between the two curves to evaluate extrinsic obstruction. Taking maximum expiratory flows as the index of airways patency, examples are shown to illustrate the use of the diagram.