Individuals in clinical training programs concerned with critical medical care must learn to manage clinical cases effectively as a member of a team. However, practice on live patients is often unpredictable and frequently repetitive. The widely substituted alternative for real patients—high-fidelity, manikin-based simulators (human patient simulator)—are expensive and require trainees to be in the same place at the same time, whereas online computer-based simulations, or virtual worlds, allow simultaneous participation from different locations. Here we present three virtual world studies for team training and assessment in acute-care medicine: (1) training emergency department (ED) teams to manage individual trauma cases; (2) prehospital and in-hospital disaster preparedness training; (3) training ED and hospital staff to manage mass casualties after chemical, biological, radiological, nuclear, or explosive incidents. The research team created realistic virtual victims of trauma (6 cases), nerve toxin exposure (10 cases), and blast trauma (10 cases); the latter two groups were supported by rules-based, pathophysiologic models of asphyxia and hypovolemia. Evaluation of these virtual world simulation exercises shows that trainees find them to be adequately realistic to “suspend disbelief,” and they quickly learn to use Internet voice communication and user interface to navigate their online character/avatar to work effectively in a critical care team. Our findings demonstrate that these virtual ED environments fulfill their promise of providing repeated practice opportunities in dispersed locations with uncommon, life-threatening trauma cases in a safe, reproducible, flexible setting.