The maximal expiratory flows and mechanical properties of the lungs were studied in 10 offspring or siblings of patients with severe deficiency of α1-antitrypsin and in 24 normal control subjects. The younger heterozygotes demonstrated no abnormality in maximal expiratory flows or lung elastic recoil. Lung elastic recoil was decreased in the heterozygotes who were more than 30 years of age. The maximal expiratory flows were less than normal in the older heterozygotes and were associated with a loss of lung elastic recoil, but the “upstream resistance” of the airways was not increased. The data suggest that the changes in lung elasticity due to aging may be accelerated in the heterozygote state.