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There are several parameters employed to evaluate lung growth or development in biopsies. The simplest method is the radial alveolar count (RAC), which is the number of alveoli transected by a perpendicular line drawn from the center of a respiratory bronchiole to the nearest septal division or pleural margin. The RAC method provides a reliable index of lung growth in intrauterine and early postnatal development, as well as in childhood. The method has been also applied in assessment of lung development in diverse pathological conditions. Up to date there are no data regarding RAC in healthy adulthood and its relation with aging. Lung specimens from CD1 mice at the age of 2, 12 or 24 months were fixed in 10% neutral-buffered formalin and paraffin-embedded. After staining of 5-µm sections with hematoxylin and eosin, RAC was determined. Results showed a statistically significant difference between the RAC means at 2 months (6.2±3.5) and 12 and 24 months (4.3±3.5 and 4.8±2.6, respectively); there was no significant difference between the means at 12 and 24 months (F= 8.61, df1= 2, df2= 259, p= 0.000; Student-Newman-Keuls test: M2≠ M12= M24). Thus, our findings show that the RAC decreases during aging until it plateau. Aging is associated with morphometric changes in the lung that lead to decreased lung function. The chronicity of this process is poorly understood with respect to time of onset or progression. This work augments our understanding of this phenomenon and demonstrates that the RAC provides a simple and accurate method to analyze aging-related changes in the lung.