The steady-state turnover of epithelial cells in the lung and trachea is highly relevant to investigators who are studying endogenous stem cells, manipulating gene expression in vivo, or using viral vectors for gene therapy. However, the average lifetime of different airway epithelial cell types has not previously been assessed using currently available genetic techniques. Here, we use Cre/loxP genetic technology to indelibly label a random fraction of ciliated cells throughout the airways of a cohort of mice and follow them in vivo for up to 18 mo. We demonstrate that ciliated airway epithelial cells are a terminally differentiated population. Moreover, their average half-life of 6 mo in the trachea and 17 mo in the lung is much longer than previously available estimates, with significant numbers of labeled cells still present after 18 mo.