ABSTRACTObjective: To build a tool to assist disaster response planning and estimate the numbers of displaced persons that will require special medical care during a disaster.Methods: We developed a tool, titled MedCon:PreEvent, which incorporates data from the 2006 National Health Interview Survey, 2005 National Hospital Discharge Survey, and 2004 National Nursing Home Survey to calculate numbers of emergency room/emergency department (ER/ED) visits, surgeries, health care home visits, overnight hospital stays, office visits, and self-rated health status. We then used thresholds of more than 12 office visits or 6 or more ER/ED visits or 6 or more surgeries or more than 4 home visits or more than 6 overnight hospital stays within the past 12 months to calculate rates per million evacuees requiring special medical care, including daily bed hospital and nursing home bed occupancy.Results: We calculated that 79 428 (95% CI = 76 940-81 770) per million evacuees would need special medical care. The daily occupation of hospital beds would be 1710 beds (95% CI = 1328-2160) per million. The occupation of nursing home beds would be 5094 beds (95% CI = 5040-5148) per million. Changing the threshold to just those who self-rated health as “poor,” the demand for special medical care would be 24 348 (95% CI = 23 087-25 535) per million. Using threshold utilization values at half the original level would increase the estimate to 226 988 (95% CI = 224 444-229 384) per million.Conclusions: A substantial number of persons with preexisting conditions will need suitable medical care following a disaster. The MedCon:PreEvent tool can assist disaster planners to prepare for medical care needs of large numbers of evacuees and consider re-evaluating the approach to utilizing and augmenting medical care services.(Disaster Med Public Health Preparedness. 2012;6:335–341)