Expression of numerous genes encoding myeloid-specific proteins is tightly regulated during normal myeloid differentiation. This heterogeneous group of genes thus offers a tool for dissection of different maturational steps of myelopoiesis. We have previously shown that the human myeloperoxidase (MPO) gene is modified at numerous CpG residues in normal myeloid cells and in myeloid cell lines in a development-specific and expression-associated manner. In the present work, we have applied this type of methylation analysis to primary leukemia myeloid cell samples. We found that expression of MPO messenger RNA (mRNA) is grossly limited to leukemias of late myeloblastic and promyelocytic stages; expression is thus associated with progressive demethylation in the 5′ region of the MPO gene. This modification was not global. Low-level induction of MPO mRNA expression in very immature leukemic cells using phorbol ester was not accompanied by progressive demethylation. This type of methylation analysis of a myeloid-specific gene may yield a molecular indicator for different types of myeloid leukemias.