Coa is a high-frequency blood group antigen in the Colton blood group system expressed on red blood cells (RBCs) of approximately 99.8 percent of random persons. Anti-Coa has been reported to cause delayed hemolytic transfusion reactions, hemolytic disease of the newborn, and accelerated clearance of RBCs in vivo. Acute hemolytic transfusion reactions (AHTRs) have not previously been reported. A 58-year-old man was hospitalized for vascular surgery. Initial blood bank evaluation revealed anti-Fya. The patient received six units of RBCs during his initial hospitalization and developed anti-E. A subsequent sample was sent to the reference laboratory when all units of RBCs appeared incompatible. Additional studies, including alloadsorptions, revealed the presence of anti-E, anti-Fya, and an apparent warm autoantibody. One unit of least-incompatible RBCs was transfused during surgery. The patient had an increase in temperature. Hemoglobinuria and a decrease in hematocrit were also noted. Due to the clinical impression of an AHTR, the pre- and postreaction samples were reevaluated in the reference laboratory and demonstrated the presence of anti-Coa in both. Based on clinical and laboratory evaluation this patient appears to have had an AHTR due to anti-Coa. This is the first known reported case of an AHTR caused by anti-Coa.