AB blood group is determined by A and B genes located on each chromosome which is inherited from parents. But unusual inheritance of A and B genes on the same chromosome has been reported as Cis AB. In some pedigree, the antigen-antibody reactions may occur despite of same AB phenotype, so called "Cis AB", in contradiction to the general Mendelian inheritance of blood group ABO expression. The anesthesiologists, often meet many cases of the transfusion, may anesthetise the Cis AB patients for surgery. Recently, authors experienced one case of patient with Cis AB blood group undergoing Cesarean Section.