The cis-AB blood group is rare; however, it is relatively more common in the Korean and Japanese populations. Among nine cis-AB alleles, only the cis-AB01 allele has been reported in the Korean population. When the A2B3 phenotype is found, it has the cis-AB01/O genotype; to date, it has not been reported in any other genotype. Here we report on an extremely rare case of cis-A2B3 found in the cis-AB01/Ax03 genotype. EDTA samples of a 52-year-old male with hepatocellular carcinoma and his family were sent to our laboratory. Standard ABO typing and sequencing of exons 6 and 7 of the ABO gene of the propositus and his family were performed: the propositus with the A2B3 type had cis-AB01/Ax03, brother with O type had O01/O02, sister with O type had Ax03/O01, son with B type had Ax03/B101, and daughter with A1B3 type had cis-AB01/A102. Results based on family study and genotyping revealed that the propositus had both cis-AB01 and Ax03 alleles. This is the first case of A2B3 phenotype with cis-AB01 with an allele other than the O allele in the cis-AB blood group reported so far.