The deposition of aerosol consisting of Teflon particles (mass median aerodynamic diameter 3.2 micrometers) labelled with 99Tcm has been measured in patients with obstructive airways disease. Aerosol was inhaled at 30 l min-1 with 4 s breath holding (patient group A) and at 30 l min-1 with 10 s breath holding (patient group B). A further group of patients (group C) inhaled at 90 l min-1 with 10 s breath holding. Each subject was given the aerosol during inhalation at three different lung volumes (20% VC, 50% VC and 80% VC) on three different days. The greatest whole lung, tracheobronchial and alveolar depositions occurred in group B patients, irrespective of the lung volume of aerosol actuation. By comparison, deposition was similar in groups A and C only when the aerosol was actuated at 20% VC. Whole lung deposition in groups A and B was significantly correlated both with the bronchodilator response to terbutaline sulphate aerosol administered under the same inhalation conditions and with the residence time of airborne particles in the respiratory tract. The optimum inhalation technique for a beta-adrenergic aerosol indicated by these data (slow inhalation with 10 s breath holding) confirms the results of previous studies.