Homozygous mutant klotho (KL(-/-)) mice exhibit various characteristics resembling those of human aging, including emphysema. However, age-related changes of lungs have not been fully elucidated. Here, we investigated the structural, functional, biochemical, and cell kinetic alterations of lungs in KL(-/-) mice at 2-12 weeks of age. Homogeneous airspace enlargement and decreased lung elastic recoil were observed in KL(-/-) mice with aging. The apoptotic cells in airway walls in KL(-/-) mice were approximately 6 times greater than those in wild-type (KL(+/+)) mice at 2 weeks of age. However, lipid peroxidation and elastase activity of lungs were not increased in KL(-/-) mice. Western blotting suggested that protein levels of epidermal growth factor (EGF) and phosphorylated extracellular signal-regulated kinase were decreased in KL(-/-) mice. These data suggest that significantly increased apoptosis of airway cells via inhibition of the EGF-dependent pathway may be involved in the development of the aging lungs in KL(-/-) mice.