In a recent analysis of a large transfusion database (Transfusion Research Utilization, Surveillance and Tracking database \[TRUST\]), the investigators found that the transfusion of ABO non-identical RBCs to group A individual was associated with an increased risk of death in-hospital compared to patients transfused with ABO identical RBCs (Red Blood Cells). Our finding was corroborated in a separate study of low birth weight neonates who received only group O RBCs (e.g., group O neonates received ABO identical RBCs but group A, B, and AB neonates received ABO non-identical RBCs). A subgroup of neonates who received ABO non-identical transfusions had higher mortality (Z. Sohl, personal communication, April 30th, 2020). Similar adverse clinical outcomes have been reported in a number of studies where patients have received ABO non-identical RBCs and/or platelets. Together, these findings raise the concern that the longstanding policy of transfusing group O non-identical RBCs and platelets may increase the risk of harm for some patients. In Hamilton, Ontario hospitals, approximately 20% of transfused patients receive ABO non-identical RBCs every year because of inventory shortages, urgent requests, and specific phenotype requirements. The negative impact of this practice could have widespread national and international implications for transfusion policy. The ability to undertake critical exploratory analyses in transfusion medicine is enabled by large research and administrative data sets that include all Hamilton hospitals. The initial finding of potential harm with ABO non-identical RBCs is hypothesis-generating and requires confirmation through external datasets and translational studies to support a biological mechanism. If confirmed, this hypothesis can then be tested in a clinical trial.