Based on preclinical data, cell-based therapy with bone marrow–derived mesenchymal stem (stromal) cells (MSCs) is a potentially attractive new therapeutic option for treating patients with the acute respiratory distress syndrome. Small and large animal models of acute lung injury from endotoxin, live bacteria, and sepsis have shown that MSCs can decrease lung injury and increase survival. The mechanisms for benefit are mediated in part by paracrine release of several antiinflammatory cytokines, keratinocyte growth factor, angiopoietin-1, as well as the release of antimicrobial peptides. There is also evidence that MSCs can transfer mitochondria and restore normal bioenergetics to injured alveolar epithelium. Some of the beneficial effects are mediated by microvesicles. A phase 1 safety and dose-escalation trial was completed and a randomized, double-blind clinical trial is currently underway.