BACKGROUND: Delayed hemolytic transfusion reactions (DHTRs) occur when pretransfusion alloantibody screening tests fail to detect previously formed alloantibodies. Hospital transfusion service records maintain historical alloantibody results. However, patients may receive transfusions at more than one hospital leading to dissociation of current and previous results. As such, we designed and implemented a regionally based registry linking patients and historic alloantibody test results with the intent of reducing DHTRs. STUDY DESIGN AND METHODS: We constructed a Web-based registry connecting hospital transfusion services and alloantibody results from a blood center immunohematology reference laboratory. Registry development required consideration of HIPAA/regulatory and Web design issues addressed by consulting legal counsel, a Web design company, and a program administrator. RESULTS: During the first year, more than 5000 patient alloantibody records were entered into the registry that were accessed more than 3900 times for 1766 patients at 68 hospitals. In four cases, registry utilization prevented a possible DHTR. CONCLUSIONS: The regional alloantibody registry prevented potential DHTRs fulfilling preimplementation goals.