A patient with Philadelphia chromosome positive CML was studied to characterize the blast transformation. Peripheral blood and bone marrow at the onset of the blast crisis were evaluated with cytochemistry, chromosome analysis, cell surface markers, terminal transferase assay, and electron microscopy. Although light microscopic examination and cytochemistry suggested lymphoblastic morphology, ultrastructural cytochemistry demonstrated the myelomonocytic features of the transformation. This study suggests that electron microscopic cytochemistry is useful in evaluating the heterogeneous nature of the blast phase of CML.