The inheritance of ABO blood type group is actually determined by triple allelic gene, A, B and O. Transmission of blood group AB by a single chromosome, instead of by two separate chromosomes, was reported and called cis AB. The anesthesiologists, who meet many cases of the transfusions, may anesthetize cis AB patients for surgery. Recently the authors have experienced one case of patient with cis AB blood type undergoing emergency craniotomy and removal of hematoma. We transfused the patient with Rh+O packed red blood cell without any significant transfusion reactions.