A single-breath, argon bolus method for measuring the lung volume at which the dependent lung zones ceased to ventilate, i.e., the closing volume, was used to study lung function in a group of 19 patients with asthma during a symptom-free state. the closing volume was measured with the patients in seated and supine positions, together with subdivisions of lung volume, one-second forced expired volume, airway resistance, and specific airway conductance. in 16 patients (84 per cent), the ratio of closing volume to vital capacity was either greater than the normal limits or there was a continuous marked increase in expired argon concentration throughout the expiration, indicating disease of the small airways. in 7 patients (37 per cent), increased closing volume or abnormality of the expired argon concentration trace was the only abnormality detected. Closing volume frequently occurred above functional residual capacity in both sitting and supine positions and could therefore cause arterial hypoxia by producing regions of low ventilation/perfusion ratios in the dependent areas of the lung. After inhalation of aerosol isoproterenol, the record of expired argon concentration became more normal in most patients, indicating that the abnormalities of ventilation distribution were partly reversible. It is concluded that many patients with asymptomatic asthma have abnormalities of ventilation distribution despite relatively normal results of routine pulmonary function tests and airway resistance. These abnormalities of ventilation distribution, which can cause impaired gas exchange within the lungs, can be reversed at least partially by bronchodilators.