Aim Major ABO incompatible bone marrow transplantation can cause Pure Red Cell Aplasia (PRCA), which can evolve into AML. We will present here incomplete reactions of missmatched donor allele(s) in relapsed AML patient with a history of ABO incompatible, haplo-identical HSCT. Methods High and low resolution HLA typing was performed by SBT and rSSO, respectively. Post-transplant engraftment analyses were conducted with STR. Results Patient is a 35 y/o, “O” blood group female with AML, who had HSCT from her “A” blood group sibling in Oct 2009. Patient’s initial HLA class I and II typing was homozygous. Her sibling was haplo-identical with 6/12 mismatch in host vs graft direction. Engraftment testing performed in Jan 2010 revealed more than 98% donor allele composition in un-fractioned whole blood, as well as in CD3+ and CD15+ populations, which was then decreased in Feb 2010 and recovered again after receiving donor lymphocyte infusion (DLI) in Aug 2010. BM biopsy of Feb 2010 revealed that 30–40% cellularity with M:E ratio greater than 10:1. Sustained erythroid hypoplasia with intact myelopoiesis and megakaryopoiesis raised a possibility of acquired PRCA. HLA typing in Jan 2010 showed full engraftment. However, following low resolution HLA verification typing performed by rSSO in Apr 2010 using buccal sample could not call mismatched alleles in HLA-A, B and DQB1 loci. In HLA-A and HLA-B loci, multiple beads presented weaker reactions than positive cut off value. To be heterozygous in DQA1 and DQB1 loci, 2 beads should be positive, however, they were too weak to be adjusted as positive. When HLA typing was tested by SBT in parallel, one DQB1 mismatched allele was still missing. Despite of DLI and other treatments, patient expired in a year after HSCT. Conclusions Summary: Both low and high resolution HLA typing methodologies revealed incomplete reactions of donor alleles (s) in relapsed AML patient, which supports other clinical findings of disease relapse.