Despite an excellent military experience with the use of the "universal donor" as an immediately available blood component, considerable reluctance to use uncrossmatched Group O packed cells (TOB) remains. In addition, problems continue with rapid blood acquisition in the emergency department. To study the safety of TOB used as an immediate resuscitation component, a 30-month prospective study of all patients arriving at a single trauma unit was undertaken. By protocol TOB (O-, female; O+, male) was delivered to the shock room prior to patient arrival and was expanded to 500 mL by adding 250 mL prewarmed saline (39.4 C) to the existing RBC unit. Transfusion was ordered on clinical signs of Class III or Class IV hemorrhage. Ninety-nine patients entered the protocol, receiving a total of 1,136 units of blood (11.5 units/patient). Four hundred ten units (4.1 units/patient) of uncrossmatched blood were administered on patient arrival--322 units of TOB and 88 units of type-specific blood (TSB). Seven patients (7.4%) had prior transfusions, and 14 (58%) women had prior pregnancies. Complications included disseminated intravascular coagulation, 12%; adult respiratory distress syndrome, 8%; and hepatitis, 1%. Forty-nine patients (49%) required massive transfusion (greater than 10 units/24 hr). All patients were followed clinically and by the blood bank for any signs of transfusion reactions or incompatibility throughout their hospital courses; none developed. There were no deaths related to transfusion incompatibility. We conclude that TOB used as an immediate resuscitative blood component is safe.(ABSTRACT TRUNCATED AT 250 WORDS)