Recent studies have shown that the anti-apoptotic Bcl-X_L, one of five isoforms expressed by the Bcl-X gene, protects a variety of cell lines exposed to hyperoxia. However, its role in lung development and protection against oxidative stress in vivo is not known. Here, we show Bcl-X_L is the predominant isoform expressed in the lung and the only isoform detected in respiratory epithelium. Since loss of Bcl-X_L is embryonic lethal, Bcl-X_L was ablated throughout the respiratory epithelium by mating mice with a floxed exon II of the Bcl-X gene with mice expressing Cre under control of the surfactant protein-C (Sftpc) promoter. Interestingly, loss of Bcl-X_L in respiratory epithelium was perinatal lethal in ~50% of the expected offspring; however, some adult mice lacking the gene were obtained. Epithelial-specific ablation of Bcl-X_L did not disrupt pulmonary function, expression of epithelial-cell specific markers, or lung development. However, it shifted the lung towards a pro-apoptotic state defined by reduction in anti-apoptotic Mcl-1, an increase in pro-apoptotic Bak, and increased sensitivity of the respiratory epithelium to hyperoxia. Intriguingly, increased 8-oxoguanine lesions seen during hyperoxia were also observed as lungs transition to room air at birth, a time when perinatal lethality of some mice lacking Bcl-X_L was observed. These findings reveal epithelial-specific expression of Bcl-X_L is not required for proper lung development, but rather functions to protect respiratory epithelial cells against oxygen-induced toxicity such as during hyperoxia and when the lung is first exposed to ambient air.