Background: Aspiration is a common cause of lung injury, but it is unclear why some cases are self-limited while others progress to ARDS. Sporadic exposure to more than one insult could account for this variable progression. We investigated whether synergy between airway acid and endotoxin (LPS) amplifies injury severity in mice and whether LPS levels in human patients could corroborate our experimental findings. Methods: C57BL/6 mice aspirated acid (pH 1.3) or normal saline (NS), followed by LPS aerosol or nothing. Lavage fluid (BALF) was obtained 2 to 49 hours later. Mice were injected with FITC-dextran 25 hours after aspiration, connected to a ventilator, and lung elastance (H) measured periodically following deep inflation (DI). Endotracheal and gastric aspirates were also collected from patients in the ICU and assayed for pH and LPS. Results: Lung instability (H following DI) and pressure-volume hysteresis in acid or LPS-exposed mice was greater than in controls, but markedly greater in the combined acid/LPS group. BALF neutrophils, cytokines, protein, and FITC-dextran in the acid/LPS mice were geometrically higher than all other groups. BALF from acid only mice markedly amplified LPS-induced TNF-α production in cultured macrophages. Human subjects had variable airway LPS levels with the highest burden in those at higher risk of aspiration. Conclusions: Acid aspiration amplifies LPS signaling in mice to disrupt barrier function and lung mechanics in synergy. High variation in airway LPS and greater airway LPS burden in patients at higher risk of aspiration could help explain the sporadic progression of aspiration to ARDS.