The goals of this study were to estimate blood group alloimmunization arising from combat-related transfusion and the prevalence of delayed serologic transfusion reactions (DSTRs) in military veteran patients. Blood group alloantibodies documented in the transfusion records at a Veterans Affairs (VA) medical center were categorized according to whether they developed before ("pre-existing") or during ("hospital-acquired") VA care and whether they were associated with anamnestic immune responses. Combat-related alloantibodies were estimated by adding anamnestic to pre-existing antibodies, revealing that 256 veterans made 322 combat-related alloantibodies. The combat-related alloimmunization rate was 1.37% (256/18,750), and combat-related alloantibodies represented 55.8% (322/577) of total alloantibodies. The highest rate of combat-related alloimmunization was observed in World War II-era veterans. Approximately 11.2% (25/224) of veterans with hospital-acquired antibodies experienced a DSTR due to prior alloimmunization. In conclusion, combat-related alloimmunization accounted for more than half of antibodies in military veterans and was a predisposing factor for DSTRs.