The Acute respiratory distress syndrome (ARDS) is a highly lethal inflammatory lung disorder. Apoptosis plays a key role in its pathogenesis. We showed that an adenovirus expressing the 70 kDa heat shock protein Hsp70 (AdHSP) protected against sepsis-induced lung injury. In this study we tested the hypothesis that AdHSP attenuates apoptosis in sepsis-induced lung injury. Sepsis was induced in rats via cecal ligation and double puncture (2CLP). At the time of 2CLP PBS, AdHSP or AdGFP (an adenoviral vector expressing green fluorescent protein) were injected into the tracheas of septic rats. 48 hours later, lungs were isolated. One lung was fixed for TUNEL staining and immunohistochemistry. The other was homogenized to isolate cytosolic and nuclear protein. Immunoblotting, gel filtration and co-immunoprecipitation were performed in these extracts. In separate experiments MLE-12 cells were incubated with medium, AdHSP or AdGFP. Cells were stimulated with TNFα. Cytosolic and nuclear proteins were isolated. These were subjected to immunoblotting, co-immunoprecipitation and a caspase-3 activity assay. TUNEL assay demonstrated that AdHSP reduced alveolar cell apoptosis. This was confirmed by immunohistochemical detection of caspase 3 abundance. In lung isolated from septic animals, immunoblotting, co-immunoprecipitation and gel filtration studies revealed an increase in cytoplasmic complexes containing caspases 3, 8 and 9. AdHSP disrupted these complexes. We propose that Hsp70 impairs apoptotic cellular pathways via interactions with caspases. Disruption of large complexes resulted in stabilization of lower molecular weight complexes, thereby, reducing nuclear caspase-3. Prevention of apoptosis in lung injury may preserve alveolar cells and aid in recovery.