© 2019 Elsevier Ltd The AABB recently posted a bulletin (19-02) regarding their recommendations for the use of group O red blood cells (RBCs) during trauma. Though group O Rh(D)-negative RBC units are considered the ‘safest’, the demand of such units often exceeds the supply. Therefore, O Rh(D)-positive units are often used during the first parts of a massive transfusion protocol (MTP) or patients with particularly severe hemorrhage are switched over from O Rh(D)-negative to O Rh(D)-positive RBC units in order to preserve the O Rh(D)-negative supply. In light of these limitations, it is important to understand the risk of such policies to the patient. The reported risk of alloimmunization after exposure to Rh(D)-positive RBCs ranges widely from 3 to 70%. In response, we performed a retrospective review of 1,198 patients in our institution that had a MTP activation due to trauma. Of those patients, we focused on Rh(D)-negative patients that received at least 1 unit of Rh(D)-positive RBCs. Seventy-two patients met the criteria for inclusion, accounting for 6% of the total population. Of the 72 Rh(D)-negative patients, we identified 17% that formed new Rh group antibodies after exposure to Rh(D)-positive RBCS. All 10 of our alloimmunized patients (two of which were females of childbearing age) formed anti-D, while 3 patients also formed either anti-E or anti-C. Since this was a retrospective review, we did not perform repeated antibody screens for the entire study period, but did review all records for the entire period. We did note that we were more likely to detect an novel alloantibody if more antibody screens were performed during the patient's initial stay and during follow-up visits. We conclude that providing Rh(D) negative patients Rh(D) positive RBC units is not without risk and policies regarding such provisions should be carefully considered. As RBC shortages continue to be a part of daily practice, such issues may continue to be a challenge for the blood bank community.