Objective: To predict how much blood will be needed based on the number of injured patients arriving after a multiple-casualty incident. Design: A retrospective study evaluating data collected in 18 consecutive terrorist attacks in the city of Tel Aviv between January 1997 and February 2005. Setting: A large, urban trauma center. Patients: A total of 986 patients in 18 events. Main Outcome Measures: Number of packed red blood cell (PRBC) units transfused per patient. Results: A total of 332 U of PRBCs were transfused. Half of the PRBC units were administered as massive transfusions to 4.7% of the patients. The number of PRBC units transfused per patient index (PPI) was related to incident size (mean [SD], 0.70[1.60] to 1.50 [1.60]). The most frequent major blood group transfused was type O (50%). Half of the units of PRBCs were supplied during the first 2 hours. Conclusions: One unit of blood per evacuated victim is sufficient in a small multiple-casualty incident and 2 U is sufficient in a large multiple-casualty incident. Half of the PRBC units should be blood group O.