BACKGROUND: Uncrossmatched type-O packed red blood cells (UORBC) are recommended for immediate transfusion in hemorrhaging trauma patients. The potential for alloimmunization with this technique is controversial, and has been reported to be as high as 80%. We examined a 1-year experience with UORBC transfusion to determine the incidence of allergic reaction and alloimmunization. METHODS: Blood Bank and Trauma Registry databases for the year 2000 were linked to determine the incidence of UORBC use and the characteristics of patients, including the incidence of transfusion reactions and seroconversion of Rh-patients. Ten units of type-O, Rh+ blood (and two units of O-blood for women of childbearing age) were available for immediate transfusion, 30 to 45 minutes sooner than type-specific or crossmatched red blood cells. UORBC were administered to any patient with signs of severe hemorrhagic shock, at the discretion of the attending physician. RESULTS: In all, 480 trauma patients (out of 5,623 admitted) received transfusions of RBC, totaling 5,203 units. Five hundred eighty-one units of UORBC were given to 161 patients. Average Injury Severity Score in the UORBC cohort was 33.8. Patients receiving UORBC received an average of 16.9 total units of red blood cells, 14 units of plasma, and 10 units of platelets. Seventy-three patients died (45%). There were no acute hemolytic transfusion reactions observed in the patients who received UORBC. Four Rh-women received UORBC, all O-. Ten Rh-men received O+ blood, and only one developed antibodies to the Rh antigen. CONCLUSION: The need for UORBC is associated with significant injury and the need for subsequent massive transfusion. In this largest reported trauma series, the use of UORBC enabled rapid administration of red cells to hemorrhaging patients, without discernible risk for transfusion-related complications. The rate of seroconversion of Rh-patients is lower than reported in the literature, perhaps due to immune suppression associated with hemorrhagic shock.