Flow-volume curves in patients with obstructive airway disease differs from that observed in healthy subjects. Two types of pathological curves can be differentiated: these with clear sharp bend and intermediate forms characterised by the different grade of concavity of the descending segments plotted against X-axis. The aim of our present investigation is to elucidate the mechanisms which determines the forced expiratory airflow course in patients with obstructive airway diseases. Patients with sharp bend curves show changes of the several lung function data which are more advanced than in subjects with the intermediate forms of the flow-volume curves. In cases of bend curves the volume of the forced expiration can be differentiated on the two parts: circumferential and serial. Circumferential volume exhaled on the very beginning of the expiration (above the bend) amounts 0.118L in average. This volume depends on the expiratory narrowing of the bronchi from the 1-st to 9-th generation. The serial volume contained between the bend and the end of expiration amounts about 95% of the expired volume. Flow limitation occurs in 5-th to 9-th generations which is manifested by the strong increase of the flow resistance. The intermediate types of the flow-volume curves is caused by the inhomogenous emptying of the lung together with corresponding volume dependent narrowing of the bronchi. The same mechanisms can be detected even on the bodypletysmographic tidal breathing resistance curves. The concave and particularly bend flow-volume curves has been attributed to the pulmonary emphysema. This is not entirely truth. Other conditions leading to inhomogenic emptying of the lung due to airway and parenchymal changes (such as lung cicatrisation) can influence expiratory flow course resulting in concave or even bend flow-volume relationships.