Our regional trauma organization, which comprises 7 trauma centers, 30 acute care hospitals and free-standing emergency departments, and 42 emergency medical services agencies, conducted possibly the largest mass-casualty drill to date, totaling 445 victims at 3 sites involving 11 hospitals and 25 agencies and organizations.The drill was preceded by a tabletop exercise 4 months beforehand called Operation Continued Care Full-Scale Exercise, which consisted of simulated terrorist events at 3 sites to wound 445 moulaged patients. Four law enforcement and 5 fire and emergency medical services departments and 16 supporting organizations and agencies were involved in transporting patients to 11 different hospitals. The 7 objectives for the event addressed coordinating emergency operations, sustaining adequate communications, updating regional bed status, processing resource requests, triaging patients, tracking patients, and patient identification.Of the 445 transported patients, 270 (60%) were entered correctly into the state patient tracking system; 68 (25.2%) upgrades and 34 (12.6%) downgrades from scene triage categories were noted. Multiple opportunities for improvement were identified, with major weaknesses noted in communication and coordination from event sites to the regional trauma organizations and hospitals.The size and complexity of the drill provided experience and knowledge to facilitate future disaster preparedness and highlighted weaknesses in communication and coordination. Large, multijurisdictional, multiagency exercises provide opportunities to stress, evaluate, and improve regional disaster preparedness.