The first patient arrived without entry notification at 3:04 p.m. by private vehicle; the patient was female and had sustained a traumatic amputation of one of her legs together with multiple other injuries. Two minutes later, a police van arrived with 2 additional patients—both also had traumatic lower extremity amputations, again there had been no time for entry notification. We activated our Hospital Incident Command System (HICS). Simultaneously, several hundred MGH staff received a phone call to their home and work, a message was also sent to their pagers, e-mail, and cell phone alerting them of the need to respond to disaster stations. The response from all staff was immediate and coordinated. Within minutes, the ED was vacated and rooms stocked in preparation for the arrival of further victims. Disaster packs, one for each expectant patient, were opened, enabling us to identify patient by prearranged medical record numbers. Preprinted wrist bands with the bar codes on them were attached to all disaster patients upon ED arrival. (We use scanned bar codes for patient identification.) Eight critical patients arrived to the hospital within 30 minutes of the explosion. Among the first was the patient who arrived pulseless; she had already exsanguinated. IVs were started; she was given 4 units of uncrossed blood and, with her blood pressure restored, was transferred immediately to the operating room. Over the next few minutes 5 other patients—3 with traumatic amputations—were also resuscitated and sent to the operating room; at that time we had positive identification on 1 of these 6 patients. More patients, albeit less severely injured, followed them to the operating room over the next 2 to 3 hours. The MGH treated 31 patients that day; several more arrived over the subsequent 24 to 48 hours.