We studied the volume of trapped gas (VTG), using a nitrogen washout method, before and after bronchodilation in four groups with theoretically increasing risk of developing pulmonary emphysema: (1) nonsmoking healthy controls (PiMn), (2) nonsmoking subjects with an intermediate alpha 1-antitrypsin deficiency (PiMZn), (3) smoking subjects with normal concentration of alpha 1-antitrypsin, and (4) smoking PiMZ subjects. VTG was the only lung function variable that showed a significant difference between PiMZn and PiMn subjects but only after bronchodilation. Some conventional lung function tests also distinguished smokers from nonsmokers of both genotypes but VTG was the most sensitive test. VTG decreased after salbutamol inhalation in the control group but showed a consecutively larger increase with more risk factors of developing emphysema. An increase in VTG after bronchodilation may be a sign of alveolar abnormality preceding development of clinical lung emphysema.