1. The forced expiratory volume (FEV0·75) was measured at increasing mouth pressures in twenty-seven patients with obstructive airways disease. Attempts were made to divide the patients on clinical grounds into emphysematous, bronchitic or asthmatic categories; there was no evidence from the (FEV/mouth pressure) plots that their airways functioned differently during forced expiration.
2. Static elastic recoil was measured in twelve patients. There was no evidence that this factor alone caused the loss of FEV in any patient.
3. It is suggested that the use of the FEV as a test of respiratory function during the natural history of obstructive airways disease should be considered in three stages. There is an initial phase when peripheral airways disease develops with little or no alteration of the FEV. In the second phase the FEV decreases from normal values to below 1·0 litre, and at this stage is considered a sensitive indicator of peripheral airways resistance. In stage 3 the FEV is low and further changes can only be small. But airways resistance continues to increase and could be a more sensitive measurement of further airways disease.