Bone marrow–derived stromal cells (BMSCs) protect against acute lung injury (ALI). To determine the role of BMSC mitochondria in this protection, we airway-instilled mice first with lipopolysaccharide (LPS) and then with either mouse BMSCs (mBMSCs) or human BMSCs (hBMSCs). Live optical studies revealed that the mBMSCs formed connexin 43 (Cx43)-containing gap junctional channels (GJCs) with the alveolar epithelia in these mice, releasing mitochondria-containing microvesicles that the epithelia engulfed. The presence of BMSC-derived mitochondria in the epithelia was evident optically, as well as by the presence of human mitochondrial DNA in mouse lungs instilled with hBMSCs. The mitochondrial transfer resulted in increased alveolar ATP concentrations. LPS-induced ALI, as indicated by alveolar leukocytosis and protein leak, inhibition of surfactant secretion and high mortality, was markedly abrogated by the instillation of wild-type mBMSCs but not of mutant, GJC-incompetent mBMSCs or mBMSCs with dysfunctional mitochondria. This is the first evidence, to our knowledge, that BMSCs protect against ALI by restituting alveolar bioenergetics through Cx43-dependent alveolar attachment and mitochondrial transfer.