Preclinical studies indicate that allogeneic human mesenchymal stem cells (MSC) may be useful for the treatment of several clinical disorders, including sepsis, acute renal failure, acute myocardial infarction, and more recently, acute lung injury (ALI). This article provides a brief review of the biologic qualities of MSC that make them suitable for the treatment of human diseases, as well as the experimental data that provide support for their potential efficacy for critically ill patients with acute respiratory failure from ALI. The article then discusses which patients with ALI might be the best candidates for cell-based therapy and provides a template for the regulatory and practical steps that will be required to test allogeneic human MSC in patients with severe ALI. There is a dual focus on how to design trials for testing both safety and efficacy.