Although the blast percentage in the bone marrow (BM) is a key parameter for the classification of myelodysplastic syndromes (MDS), the current blast percentages used to define MDS subtypes have not been shown to have strong biological relevance. We determined the blast phenotypes and examined their relationship with the BM blast percentage in 90 MDS cases. When the BM blast percentage increased, cases whose blasts expressed CD7, CD56 and CD117 increased whereas cases whose blasts expressed CD10, CD11b and CD15 decreased. The BM blast percentages where the blast immunophenotype changed were 5, 10, 20 and 25%. Blast immunophenotypes have the potential to provide a biological basis for and refine the present MDS classifications.