AbstractTransformation of myelodysplastic syndrome to hematologic malignancies is not uncommon, but involvement of three committed progenitor cell lines at one time is seldom seen. We present a case of myelodysplastic syndrome with blast transformation, neoplastic cells that were difficult to classify morphologically, and basophilia found in marrow and peripheral blood. Differential diagnosis included myeloblastic leukemia, erythroblastic leukemia, megakaryoblastic leukemia, basophiloblastic leukemia, and systemic mast cell disease (SMCD). By using a battery of cytochemical techniques niques and irnrnuno~tochemicalm ethods, we were able to demonstrate the presence of myeloblasts, erythroblasts, and megakaryoblasts, thus excluding the possibility of SMCD and basophiloblastic leukemia. (The J Histotechnol13:67, 1990)