Increasing evidence suggests that high case volume is associated with improved outcomes in the intensive care unit (ICU). Potential explanations for the volume-outcome relationship include selective referral, clinical experience and organizational factors common to high-volume ICUs. Distinguishing between these explanations has important health policy implications, because outcomes at low-volume ICUs could be improved either by exporting best practices found at high-volume centers or by regionalizing adult critical care - two very different care strategies. Future research efforts should be directed at better characterizing the process of care in high-volume ICUs and exploring the feasibility of creating a regionalized system of care.