Acid aspiration, which can result from several etiologies, including postoperative complications, leads to direct contact of concentrated hydrochloric acid (HCl) with the alveolar epithelium. As a result, rapid endothelial activation induces alveolar inflammation, leading to life-threatening pulmonary edema. Because mechanisms underlying the rapid endothelial activation are not understood, here we determined responses in real time through optical imaging of alveoli of live mouse lungs. By alveolar micropuncture, we microinfused concentrated HCl in the alveolar lumen. As expected, acid contact with the epithelium caused rapid, but transient, apical injury. However, there was no concomitant membrane injury to the endothelium. Nevertheless, H 2 O 2-mediated epithelial-endothelial paracrine signaling induced endothelial barrier failure, as detected by microvascular dextran leakage and lung water quantification. Remarkably, endothelial mitochondria regulated the barrier failure by activating uncoupling protein 2 (UCP2), thereby inducing transient mitochondrial depolarization that led to cofilin-induced actin depolymerization. Knockdown, or endothelium-targeted deletion of UCP2 expression, blocked these responses, including pulmonary edema. To our knowledge, these findings are the first to mechanistically implicate endothelial mitochondria in acid-induced barrier deterioration and pulmonary edema. We suggest endothelial UCP2 may be a therapeutic target for acid-induced acute lung injury.