Many studies have demonstrated that closed intensive care units (ICUs), staffed by trained intensivists, are associated with improved patient outcomes. However, the mechanisms by which ICU organizational factors, such as physician staffing, influence patient outcomes are unclear. One potential mechanism is the increased utilization of evidence-based practices in closed ICUs. Cooke and colleagues investigated this hypothesis in a cohort of 759 acute lung injury patients in 23 ICUs in King County, Washington, USA. Although closed ICUs were independently associated with a modestly lower mean tidal volume, this finding did not explain the mortality benefit associated with a closed ICU model in this patient cohort. Future studies should evaluate other potential mechanisms by which closed ICUs improve patient outcomes. An improved understanding of these mechanisms may yield new targets for improving the quality of medical care for all ICU patients.